Added proto file for the executor API (#7057)

What changed?
Added protofile and generated files for the shard distributor executor API

The executor is the service processing the shards.

The interface has a single endpoint "heartbeat" where we send the current state, and receive the current assignment from the shard distributor service

Why?

Part of the shard distributor work

How did you test it?
Tested proto could generate the go code from the files

Potential risks
None, just adds an internal API spec

Release notes

Documentation Changes

Author: jakobht