Support Reset Connection as a first class exception in api_core

[frankyn]

Is your feature request related to a problem? Please describe.
Users are continuously experiencing reset connection errors which are retried in https://github.com/googleapis/google-api-python-client/releases/tag/v1.8.1. At the moment the error needs to be handled outside of the api_core package instead of being part of one of the supported exceptions in https://github.com/googleapis/python-api-core/blob/master/google/api_core/exceptions.py#L336 and surfaces transport library types when it should be wrapped.

Describe the solution you'd like
Include Reset Connection as a first class citizen in https://github.com/googleapis/python-api-core/blob/master/google/api_core/exceptions.py#L336

Describe alternatives you've considered
Including reset connection into library retry code manually for each manual library. It can instead be handled generally in api_core instead.

cc: @crwilcox @busunkim96 @tritone @shollyman

[frankyn]

Discussed offline with the group:

The changes necessary to support this error:

1. Update api-core to support the ResetConnection error as an exception class.
2. Update cloud-core to catch the underlying HTTP reset connection error and wrap it as a ResetConnection error defined in api-core.

This is only true while the libraries are split into two parts.