Remote Execution debugging operations built into buck2

[alexlian]

Per #622 referencing an internal Meta tool, we should provide proper RE functionality without a separate binary. Separating out the issue to correctly align title and close out the old one.

Operations should include: buck2 re download-action, buck2 re download-blob, etc. That would let everyone have access to this functionality in a backend independent way.

Note, this pathway should not be allowed for automation in general.

A possible substitution, and likely RBE providers may offer their own: https://github.com/bazelbuild/remote-apis-sdks/blob/master/go/cmd/remotetool/main.go>