* Install `buf` instead of `protoc`
* Created `buf.yaml` and `buf.gen.yaml` files in the two proto directories to control how `buf` generates/lints proto code.
* Invoke `buf` instead of `protoc`
* Added a `proto-format` make target.
* Committed the reformatted proto files.
* Added a `proto-lint` make target.
* Integrated proto linting with CI
* Fixed tons of proto linter warnings.
* Got rid of deprecated builtin protoc-gen-go grpc plugin usage. Moved to direct usage of protoc-gen-go-grpc.
* Unified all proto directories / go packages around using pb prefixes but ensuring all proto packages do not have the prefix.
Most of the groundwork was laid in previous PRs between adding the cert-monitor package to extracting the logic of signing certificates out of the connect_ca_endpoint.go code and into a method on the server.
This also refactors the auto-config package a bit to split things out into multiple files.
