feat(shard-distributor): add canary gRPC protocol for executor-to-executor pings (#7475)

**What changed?**
Added `ShardDistributorExecutorCanaryAPI` service with a `Ping` RPC
method. Includes proto definition and generated Go code.

**Why?**
Enable canary testing to verify executor-to-executor communication and
shard ownership accuracy. The Ping RPC allows one executor to ping
another that owns a specific shard, validating:
- Executors can route to each other based on shard ownership
- Shard ownership information is accurate
- The shard distributor is functioning correctly

**How did you test it?**
Proto compiles successfully and generates valid Go code. Will be used by
canary components in follow-up PRs.

**Potential risks**
None - this only adds a proto definition with no implementation yet.

**Release notes**


**Documentation Changes**
None

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

Author: jakobht