Python's external_pb2.py incompatible with actually-used external.proto

[i-infra]

Describe the bug

external_pb2.py needs to be updated.

To Reproduce

Try and decode a receiver receipt object using external_pb2.py from this repo.

Expected behavior

Receiver receipt successfully decoded.

Actual behavior

Error thrown as fields have changed for block version 2.

Environment (please complete the following information):

• OS: Ubuntu
• Version 20.04

Additional context

Fixable with protoc --python_out=/tmp/mc_py ./mobilecoin/api/proto/external.proto

I'll probably make some time to MR the fix "soon".