Add bigdecimal to runtime dependencies (#1367)

Ruby 3.4 will promote bigdecimal gem to a bundled gem in order to improve maintenancebility.
To migrate to Ruby 3.4 easily, Ruby 3.3 warns a use of bigdecimal without adding it to dependencies.

> /build/lib/mysql2.rb:2: warning: bigdecimal was loaded from the standard library, but will no longer be part of the default gems since Ruby 3.4.0. Add bigdecimal to your Gemfile or gemspec.

So this patch adds bigdecimal to runtime dependencies.

I know bigdecimal is not always necessary, but I'd like to prevent a lot of users from adding the gem to their Gemfile manually.

Ref. https://bugs.ruby-lang.org/issues/20187

mysql2.gemspec

@@ -22,4 +22,6 @@ Mysql2::GEMSPEC = Gem::Specification.new do |s|
   s.files = `git ls-files README.md CHANGELOG.md LICENSE ext lib support`.split
 
   s.metadata['msys2_mingw_dependencies'] = 'libmariadbclient'
+
+  s.add_runtime_dependency 'bigdecimal'
 end