feat(query): GRPC call accepting an execution plan and corresponding dispatcher

Pull Request checklist

[X] The commit(s) message(s) follows the contribution guidelines ?
[ ] Tests for the changes have been added (for bug fixes / features) ?
[ ] Docs have been added / updated (for bug fixes / features) ?

Current behavior : (link exiting issues here : https://help.github.com/articles/basic-writing-and-formatting-syntax/#referencing-issues-and-pull-requests) It is impossible to send an execution plan to a query service

New behavior : One can send an execution plan to a query service using a GRPC call.

BREAKING CHANGES

The proto definitions of execution plans had been moved to a different namespace. This should not be an issue for migration for as long as the proto execution plans are not used.

Other information:

filodb / FiloDB

feat(query): GRPC call accepting an execution plan and corresponding dispatcher #1748