Add support for HTTP PATCH method on `BaseClient`

[bjmc-globus]

We already support GET, PUT, POST and DELETE, but we should add support for PATCH, to make it easier to build a client that integrates against an API that uses that verb.

[jimPruyne]

I actually have the (trivial) update to add it to the BaseClient but finding suitable test cases was not obvious to me. As far as I know, the only API we have that uses PATCH is the Identifiers service, and I was hesitant to add dependencies on more services to the SDK test suite, particularly since the SDK doesn't yet support Identifiers in general.

Perhaps we roll this in when we roll in support for Identifiers more generally?