**What does this PR do?**

This PR fixes an issue that we identified in the
`profiling/tasks/setup_spec.rb` that was triggered by applying the
at_fork monkey patch before running that spec.

This issue could be triggered by adding

```ruby
Datadog::Core::Utils::AtForkMonkeyPatch.apply!
```

to the top of the `setup_spec.rb` (before the `RSpec.describe`).

```
Datadog::Profiling::Tasks::Setup
  #run
    actives the forking extension before setting up the at_fork hooks
    only sets up the extensions and hooks once, even across different instances
  #setup_at_fork_hooks
    when Process#datadog_at_fork is available
      sets up an at_fork hook that restarts the profiler (FAILED - 1)
      when there is an issue starting the profiler
        logs an exception (FAILED - 2)
        does not raise any error (FAILED - 3)
    when #datadog_at_fork is not available
      logs a debug message

Failures:

  1) Datadog::Profiling::Tasks::Setup#setup_at_fork_hooks when Process#datadog_at_fork is available sets up an at_fork hook that restarts the profiler
     Failure/Error:
       example.run.tap do
         tracer_shutdown!
       end

     NameError:
       undefined method `datadog_at_fork' for class `#<Class:Process>'

               object_singleton_class.__send__(@original_visibility, method_name)
                                     ^^^^^^^^^
     # ./spec/spec_helper.rb:230:in `block (2 levels) in <top (required)>'
     # ./spec/spec_helper.rb:115:in `block (2 levels) in <top (required)>'
     # webmock-3.23.1/lib/webmock/rspec.rb:39:in `block (2 levels) in <top (required)>'
     # rspec-wait-0.0.10/lib/rspec/wait.rb:47:in `block (2 levels) in <top (required)>'

  2) Datadog::Profiling::Tasks::Setup#setup_at_fork_hooks when Process#datadog_at_fork is available when there is an issue starting the profiler logs an exception
     Failure/Error: at_fork_hook.call

     NoMethodError:
       undefined method `call' for nil:NilClass

                 at_fork_hook.call
                             ^^^^^
     # ./spec/datadog/profiling/tasks/setup_spec.rb:84:in `block (5 levels) in <top (required)>'
     # ./spec/spec_helper.rb:230:in `block (2 levels) in <top (required)>'
     # ./spec/spec_helper.rb:115:in `block (2 levels) in <top (required)>'
     # webmock-3.23.1/lib/webmock/rspec.rb:39:in `block (2 levels) in <top (required)>'
     # rspec-wait-0.0.10/lib/rspec/wait.rb:47:in `block (2 levels) in <top (required)>'

  3) Datadog::Profiling::Tasks::Setup#setup_at_fork_hooks when Process#datadog_at_fork is available when there is an issue starting the profiler does not raise any error
     Failure/Error: at_fork_hook.call

     NoMethodError:
       undefined method `call' for nil:NilClass

                 at_fork_hook.call
                             ^^^^^
     # ./spec/datadog/profiling/tasks/setup_spec.rb:76:in `block (5 levels) in <top (required)>'
     # ./spec/spec_helper.rb:230:in `block (2 levels) in <top (required)>'
     # ./spec/spec_helper.rb:115:in `block (2 levels) in <top (required)>'
     # webmock-3.23.1/lib/webmock/rspec.rb:39:in `block (2 levels) in <top (required)>'
     # rspec-wait-0.0.10/lib/rspec/wait.rb:47:in `block (2 levels) in <top (required)>'
```

This issue was caused by an incompatibility between rspec-mocks (not
entirely sure if it's a bug) and the monkey patching we were doing.

Rather than fighting with more monkey patch weirdness or rspec,
I decided to fix the issue by simplifying the at fork monkey patch
module by making it as minimal as possible:

* It no longer places extra unneeded functions in Ruby classes, e.g.
  `datadog_at_fork_blocks` and `datadog_at_fork`
* It no longer stores its data in a weird class var

This enabled a bunch of simplifications, and the affected
spec was fixed as a side-effect of this.

**Motivation:**

Get the at_fork monkey patch in shape to be used by the crashtracker as
well.

**Additional Notes:**

I was tempted to do a larger refactoring of this module when I was
extracting it, but I decided to aim for a smaller diff.

Since I needed to dive in again to fix the issue with the spec, I
decided to finally go for it.

**How to test the change?**

These changes include test coverage.

Here's a tiny app that can be used to experiment with this:

```ruby
puts RUBY_DESCRIPTION

require 'datadog/core/utils/at_fork_monkey_patch'

Datadog::Core::Utils::AtForkMonkeyPatch.apply!

Datadog::Core::Utils::AtForkMonkeyPatch.at_fork(:child) { puts "Hello from child process: #{Process.pid} "}

puts "Parent pid: #{Process.pid}"

puts "fork { }"
fork { }
Process.wait

puts "Kernel.fork { }"
Kernel.fork { }
Process.wait

puts "Process.fork { }"
Process.fork { }
Process.wait

puts "Foo.new.call"
class Foo
  def call
    fork { }
  end

  def self.do_fork
    fork {  }
  end
end
Foo.new.call
Process.wait

puts "Foo.do_fork"
Foo.do_fork
Process.wait

puts "Fork from outside"
Foo.new.send(:fork) { }
Process.wait

class BasicFoo < BasicObject
  include ::Kernel

  def call
    fork { }
  end

  def self.do_fork
    fork { }
  end
end

puts "BasicFoo.new.call"
BasicFoo.new.call
Process.wait

puts "BasicObject:"
Class.new(BasicObject) { include(::Kernel); def call; fork { }; end }.new.call
Process.wait

puts "BasicFork.do_fork"
BasicFoo.do_fork
Process.wait
```

…-patch