api-linter as a protoc plugin, e.g., protoc-gen-api-linter

Thanks for stopping by to let us know something could be better!

**Is your feature request related to a problem? Please describe.**

I'd like to reuse the functionalities of bazel's `proto_library` for managing dependencies and `import` paths.
After some debugging and trying out ideas, I find plugins specifically usable and useful and easy to integrate into existing Starlark rules.
Hence this feature request.

**Describe the solution you'd like**
In addition to `//cmd/api-linter`, provide a `protoc-gen-api-linter`.

In a manner similar to how [`protolint`](https://github.com/yoheimuta/protolint) provides a `protoc-gen-protolint` plugin (https://github.com/yoheimuta/protolint/blob/master/cmd/protoc-gen-protolint/README.md).

**Describe alternatives you've considered**
Well, the real alternative worth comparing is just `api-linter` as is. I am happy with it for the most part, except that I have to work hard to find out proper `-I` flags and order them correctly.

**Additional context**
None that I'm aware of right now. But could add more later.