bitbucketcloud: add required client methods for Batch Changes
Created by: LawnGnome
This PR extends internal/extsvc/bitbucketcloud.Client
to implement the methods that we need to support Batch Changes.
It looks giant, but it's not really that bad — about three quarters of the diffstat is auto-generated test cases.
Test plan
All new client methods have unit tests, verified by generating coverage stats — the only lines not covered are unlikely error branches where we don't do anything but bail out anyway.
This has also received more ad hoc integration testing via the omnibus PR this is split from.