Adds new tests in `//compatibility` for detecting undesired breaking changes in the schema of the well-known types and `descriptor.proto` files, using the Buf breaking change detector, via [`rules_buf`](https://github.com/bufbuild/rules_buf/).
In order to keep things light-touch as far as maintenance goes, I have chosen to keep the integration as small and simple as possible. Some notes:
- Breaking change behavior can be granularly controlled via [`buf.yaml`](https://buf.build/docs/configuration/v1/buf-yaml#breaking).
- Bazel sandboxes us away from meaningful VCS information, so in order to pick a target to check for breaking changes against, a new variable is added to `protobuf_versions.bzl` that needs to be updated with changes in the release version.
- Breaking change detection is performed on a file-level, not a package-level, so migrating types across WKT files would constitute a breaking change. If this is not desired the behavior can be made to work on a package-level, though we need to do some more work as `buf_breaking_test` currently only accepts a single file descriptor set target for the `against` attribute.
Closes#17513
jchadwick-buf