Cluster server client: stub creation depends on stub

[dmitrii-ubskii]

Description

Potential circular dependency in https://github.com/vaticle/typedb-client-python/blob/master/typedb/connection/cluster/server_client.py.

The constructor of _ClusterServerClient calls _ClusterServerClient.new_channel_and_stub() during creation of the channel and the gRPC _stub of a server client (L36),
which calls _ClusterServerClient._new_channel() (L49),
which references a yet-not-created self._stub in a lambda that it passes to grpc.metadata_call_credentials() (L60),
which, if it calls the lambda at that point, correctly raises AttributeError: '_ClusterServerClient' object has no attribute '_stub'.

[alexjpwalker]

Hopefully this issue will dissolve when we call Rust via FFI.