Add a buf.yaml for Buf CLI compatibility

[Alfus]

To control which files get synced to https://buf.build/cel/spec.

[Alfus]

Alternatively, this could be managed by buf.

[TristonianJones]

@Alfus does this help buf.build users more easily integrate with CEL?

[TristonianJones]

/gcbrun

[Alfus]

This does allow the buf CLI to run in the cel-spec repo, which allows for local code gen, linting, etc. However, the real reason to include this file, is to control which .proto file descriptors are uploaded to buf.build/cel/spec. Once uploaded, anyone using the buf CLI can access them remotely either by referencing buf.build/cel/spec in a command line argument, or add it as a dependency to a buf.yaml, in a different repo.