feat/enterpriseportal: RevokeEnterpriseSubscriptionLicense RPC definition (#63331)

Enterprise Portal equivalent of the `revokeLicense` GraphQL mutation.
It's not a "delete" operation - we currently retain the record forever -
so it's idempotent, as the revocation of an already-revoked license 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.

Related: https://github.com/sourcegraph/sourcegraph/pull/63330

Closes CORE-153

## Test plan

CI

Author: bobheadxi