This PR migrates from Protobuild (which we all love and use for quite some time) to buf to manage our proto files.

Immediate benefits:

• No need to install protoc dependency. Mush simpler CI setup.
• Much better local/CI reproducibility - all generators and dependencies are pinned in buf.yaml and buf.lock, so same output is expected everywhere. Only the buf binary need to be installed on the system to get things going.
• No longer needs containerd to be in GOPATH (not strictly buf’s feature, but implemented in this PR)

There are also some longer term nice-to-have features that we could benefit from, which we don't have in protobuild:

• Breaking change detector (we can run this on CI to guaranty API compatibility)
• Linter
• Formatter.

I was able to generate exactly the same code with buf as it was before.
The only annoying thing is

// protoc (unknown)

which seems to be expected when buf is not using external protoc binary (which we don't):

The protoc (unknown) line is being inserted by protoc-gen-go which we do not control. Part of the CodeGeneratorRequest passed to protoc-gen-go specifies the version of protoc being used, but buf is not, and doesn't use, protoc, so there is no appropriate answer here.

Made a few follow up changes based on feedback:

• Switched to relative imports (which buf supports natively), so workarounds in the Makefile no longer necessary.
• Moved buf configuration files under api/ directory