Provide stubs/annotations for RecordWrapper

Currently there's no way for a user to know what EPICS record attributes are accessible on a `RecordWrapper` without just trying to get the attribute - and only a small handful are accessible at the moment. This would also help static type checkers such as `mypy` to better reason about the values retrieved from the record.

A basic introduction to stubs can be found here: https://mypy.readthedocs.io/en/stable/stubs.html