* 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.
Introduces a gRPC endpoint for signing Connect leaf certificates. It's also
the first of the public gRPC endpoints to perform leader-forwarding, so
establishes the pattern of forwarding over the multiplexed internal RPC port.
Adds a new gRPC streaming endpoint (WatchRoots) that dataplane clients will
use to fetch the current list of active Connect CA roots and receive new
lists whenever the roots are rotated.
Kyle Havlovitz