Split and move proto defintions

Splits the proto service definition in a version with only unary rpcs `sql_command_service.proto` and `sql_command_service_experimental.proto` in which we can continue development/experiments/tests with streaming rpcs. `sql_command_service.proto` is a strict subset of `sql_command_service_experimental.proto` and the intersection of the commands are compatible with each other. In places where we are not restricted to old ScalaPB, we can use `sql_command_service_experimental.proto`.

The request/response types have been moved to a separate file `messages.proto` that both service files import (to aid in keeping them compatible and reduce duplication). I've also created top-level types for the responses, instead of nesting them in the request types, as this is not standard practice for gRPC.

Finally, this moves the protos to the top-level cmdexec folder.

Regular tests / manual test with local server and `CLITestClient` / python client unit tests

Author: Sander Goos <sander.goos@databricks.com>

Author: Ubuntu

…lients/python/sql_command_service_pb2.py …dexec/clients/python/api/messages_pb2.pycmdexec/clients/python/sql_command_service_pb2.py renamed to cmdexec/clients/python/api/messages_pb2.py cmdexec/clients/python/sql_command_service_pb2.py renamed to cmdexec/clients/python/api/messages_pb2.py

@@ -0,0 +1,3 @@
+# Generated by the gRPC Python protocol compiler plugin. DO NOT EDIT!
+"""Client and server classes corresponding to protobuf-defined services."""
+import grpc