cmd/buildlet: add healthz endpoint
cmd/buildlet: add healthz endpoint

This adds a healthz endpoint to buildlets. For reverse buildlets, it
also listens for healthz requests on a private port for a monitoring
process.

For golang/go#47018

Change-Id: I100a8939c5752664afb80472e567ab05a80649d7
Reviewed-on: https://go-review.googlesource.com/c/build/+/334373
Trust: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
Run-TryBot: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/build,golang/build,golang/build,golang/build