Add bazel targets for standalone plugin binaries

[bufdev]

Follow-up to https://github.com/protocolbuffers/protobuf/pull/23736 and https://github.com/protocolbuffers/protobuf/pull/13308 but with only Bazel targets added per feedback.

This PR adds targets to build protoc plugins for all the built-in plugins. This will result in the following binaries being available:

• protoc-gen-cpp
• protoc-gen-csharp
• protoc-gen-java
• protoc-gen-kotlin
• protoc-gen-objectivec
• protoc-gen-php
• protoc-gen-python
• protoc-gen-pyi
• protoc-gen-ruby
• protoc-gen-rust