Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>

In this PR:
- Add a CLI tool to validate generation config.

Downstream libraries, e.g., google-cloud-java, can write a workflow job
like:
```
docker run ...
 python /src/cli/entry_point.py \
 validate-generation-config \
 --generation-config-path=path/to/generation_config.yaml
```

🤖 I have created a release *beep* *boop*
---


<details><summary>2.40.0</summary>

##
[2.40.0](googleapis/sdk-platform-java@v2.39.0...v2.40.0)
(2024-05-02)


### Features

* [common-protos] add `Weight` to common types for Shopping APIs to be
used for accounts bundle
([#2699](googleapis/sdk-platform-java#2699))
([90a28ef](googleapis/sdk-platform-java@90a28ef))
* add a CLI tool to validate generation configuration
([#2691](googleapis/sdk-platform-java#2691))
([f52f8cc](googleapis/sdk-platform-java@f52f8cc))
* Parser to consume the api-versioning value from proto
([#2630](googleapis/sdk-platform-java#2630))
([c112b37](googleapis/sdk-platform-java@c112b37))
* Update Gapic generator and Gax to emit api-versioning via header
([#2671](googleapis/sdk-platform-java#2671))
([cb5ada0](googleapis/sdk-platform-java@cb5ada0))


### Bug Fixes

* change folder prefix for adding headers
([#2688](googleapis/sdk-platform-java#2688))
([ab5ba1d](googleapis/sdk-platform-java@ab5ba1d))
* Log HttpJson's async thread pool core size
([#2697](googleapis/sdk-platform-java#2697))
([31038d1](googleapis/sdk-platform-java@31038d1))
* replace `cfg = "host"` with `cfg = "exec"`
([#2637](googleapis/sdk-platform-java#2637))
([fd12e0f](googleapis/sdk-platform-java@fd12e0f))
* Return resolved endpoint from StubSettings' Builder
([#2715](googleapis/sdk-platform-java#2715))
([6139a73](googleapis/sdk-platform-java@6139a73))


### Dependencies

* Make opentelemetry-api an optional dependency.
([#2681](googleapis/sdk-platform-java#2681))
([2239c8c](googleapis/sdk-platform-java@2239c8c))
* update dependency absl-py to v2.1.0
([#2659](googleapis/sdk-platform-java#2659))
([b2b7738](googleapis/sdk-platform-java@b2b7738))
* update dependency gitpython to v3.1.43
([#2656](googleapis/sdk-platform-java#2656))
([6d273ad](googleapis/sdk-platform-java@6d273ad))
* update dependency lxml to v5.2.1
([#2661](googleapis/sdk-platform-java#2661))
([7a34168](googleapis/sdk-platform-java@7a34168))
* update dependency net.bytebuddy:byte-buddy to v1.14.14
([#2703](googleapis/sdk-platform-java#2703))
([72b0441](googleapis/sdk-platform-java@72b0441))
* update dependency typing to v3.10.0.0
([#2663](googleapis/sdk-platform-java#2663))
([4c64e98](googleapis/sdk-platform-java@4c64e98))
* update gapic-showcase to v0.33.0
([#2653](googleapis/sdk-platform-java#2653))
([5c12155](googleapis/sdk-platform-java@5c12155))


### Documentation

* Add contributing guidelines to PR and issue templates
([#2682](googleapis/sdk-platform-java#2682))
([82f5ce8](googleapis/sdk-platform-java@82f5ce8))
</details>

---
This PR was generated with [Release
Please](https://github.com/googleapis/release-please). See
[documentation](https://github.com/googleapis/release-please#release-please).

Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>

…ier#cachedPool a singleton (#2691)