batches: add a username field for code host credentials that require one
Created by: LawnGnome
We need this for Bitbucket Cloud, which always requires a username and password pair. Right now this is an invisible change, since the GraphQL call to get Batch Changes code hosts will never return Bitbucket Cloud instances.
Test plan
This has normal storybook coverage for a new code host type, and has been tested in conjunction with the other Bitbucket Cloud PRs end to end.