Add new 'mock' feature for 'mock-mount-s3' binary

[dannycjones]

Description of change

Without this change, we get a compilation error due to missing dependencies:

   Compiling mountpoint-s3-crt v0.9.0 (/Users/djonesoa/devel/mountpoint-s3/mountpoint-s3-crt)
error[E0432]: unresolved import `futures::executor::ThreadPool`
   --> mountpoint-s3/src/bin/mock-mount-s3.rs:13:5
    |
13  | use futures::executor::ThreadPool;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no `ThreadPool` in `executor`
    |
note: found an item that was configured out
   --> /Users/djonesoa/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-0.3.30/src/lib.rs:203:32
    |
203 |     pub use futures_executor::{ThreadPool, ThreadPoolBuilder};
    |                                ^^^^^^^^^^
    = note: the item is gated behind the `thread-pool` feature

For more information about this error, try `rustc --explain E0432`.
error: could not compile `mountpoint-s3` (bin "mock-mount-s3") due to 1 previous error

This change adds a new mock feature to the mountpoint-s3 crate, which implies both mountpoint-s3-client/mock and futures/thread-pool. We mark the mock-mount-s3 client as requiring the mock feature, so building will fail as follows:

❯ cargo run --bin mock-mount-s3 -- sthree-test ~/mnt/s3                
error: target `mock-mount-s3` in package `mountpoint-s3` requires the features: `mock`
Consider enabling them by passing, e.g., `--features="mock"`

By building with the feature enabled, building completes.

❯ cargo build --bin mock-mount-s3 --features mock   
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.22s

Relevant issues: #1018

Does this change impact existing behavior?

It changes the features required to build the mock-mount-s3 binary. This is not stable.

Does this change need a changelog entry in any of the crates?

No, this is not customer facing behavior.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and I agree to the terms of the Developer Certificate of Origin (DCO).

[monthonk]

LGTM. Happy to merge after the conflict is resolved.