Add canary gRPC protocol for executor-to-executor pings

Define ShardDistributorExecutorCanaryAPI service that allows one executor
to ping another executor that owns a specific shard. This is used for
canary testing to verify that:
1. Executors can route to each other based on shard ownership
2. Shard ownership information is accurate
3. The shard distributor is functioning correctly

The Ping RPC takes a shard key and namespace, and returns whether the
receiving executor owns that shard along with the executor ID.

Signed-off-by: Jakob Haahr Taankvist <[email protected]>

Author: jakobht