Add unit tests that exercise all the validation cases and assert on the
k8s `ErrorMatcher` instead of comparing raw error strings.
Moved most validations from functional tests to unit tests, and kept one
validation for each gRPC method to validate that handlers properly map
Go errors to gRPC statuses.
Replace actor_id and atespace name with `metadata` field which
holds common fields.
This commit only changes the ate-apiserver to support this field, but
intentionally skips upstream / downstream systems (such as kubectl-ate,
atenet, atelet, etc) to keep the scope manageable. This will come in subsequent PRs.
Julian Gutierrez Oschmann