Enable api-linter on buf

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**Is your feature request related to a problem? Please describe.**
The API linter is a useful way of ensuring API protobuf definitions are AIP-compliant. [buf](buf.build) is gradually becoming an industry standard for publishing and managing protobufs, but today: 1) There is no way of natively running aip-linter in buf, and 2) The buf default lint rules contradict AIP lint rules. For example, buf recommends that no two RPC methods return the same message, but AIP recommends `Get` and `Create` methods both return the same message.

**Describe the solution you'd like**
Native aip-linter integration into buf as a plugin.

**Describe alternatives you've considered**
Run aip-linter entirely separately, or don't use it at all, if using buf.

**Additional context**
N/A