feat/enterpriseportal: ArchiveEnterpriseSubscription RPC definition (#63330)

Enterprise Portal equivalent of the `archiveProductSubscription` GraphQL
mutation. It's not a "delete" operation - we retain the record forever -
so it's idempotent, as an archive on an already-archived subscription
can return OK. If we want to add a hard delete we should add a separate
one based more closely on https://google.aip.dev/135.

It's not part of "update" because it's closer to a "delete" than
anything else, and currently cannot be undone.

Closes CORE-153

## Test plan

CI

Author: bobheadxi