Remote-execution instance configuration support (technical debt)

Background

Initial remote-execution implementation (!626 (merged)) does not support multi-instance servers and always sends requests not specifying any target instance. We need to be able to set the instance we target for the build command we send.

One way to do this (others may be discuses here) would be to extend the configuration from this:

remote-execution:
  url: http://localhost:50051

To something like this:

remote-execution:
  url: http://localhost:50051
  instance: build

Task description

Implementation should include:

• Add an instance configuration key to the remote-execution section of project.conf.
• Ensure we read that configuration and send requests specifying that instance name.