Add support for Remote Execution API

[tagantroy]

Is your feature request related to a problem? Please describe.
Building test farm is not very trivial problem, you can start with using open sourced tools like DeviceFarmer(ex OpenSTF), but it will solve only part of your challenges.
Google introduced Remote Execution API to support horizontal scaling of their build system Bazel, but this API can be easily adopted to for many things. One of them - run tests.

Remote Execution API covers most of important topics:

1. Delivering build artifacts to workers
2. Executing command on remote workers
3. Providing access to execution results

Describe the solution you'd like

Implement new vendor module using RE API for Android(and iOS in the future).

1. RE API Protobuf specification
2. RE API Design doc

Describe alternatives you've considered

1. Remote Worker API, also introduced by Google. Design doc
2. SSH based implementation

Additional context
Open sourced implementations of Remote Execution API:

• bazel-remote (open source, cache only)
• Buildbarn (open source)
• BuildBuddy (commercial & open source)
• Buildfarm (open source)
• BuildGrid (open source)

Open sourced implementations of Remote Worker API:

• BuildGrid (open source)