Add ArrayResponse and use it for GroupsClient.get_my_groups

[sirosen]

I noticed some code out in the wild doing

res = client.get_my_groups()
return {x["id"] for x in res.data}

Naturally, we'd like to be able to write this inline as

return {x["id"] for x in client.get_my_groups()}

but the response object is not iterable.

The overall goal of this PR is to enable the above style of usage by adding response.ArrayResponse and using that in GroupsClient.get_my_groups. ArrayResponse should support __iter__ and __getitem__ at least, arguably __len__ as well.

Along the way, I've checked for dunder methods other than __iter__ which are missing and which make sense to implement on the ArrayResponse type. Some interesting things:

• __bool__ is reasonable in the normal cases, and even makes sense on dict data. You could take a very strict line and consider its introduction breaking because previously bool(response) was always True. I think that's far too strict, and have included __bool__
• __getitem__ needs changes. At first, I thought ArrayResponse.__getitem__ would be specialized, but the base response class doesn't claim to not handle arrays. So __getitem__ now accepts str | int | slice in its annotation.
• __len__ looks like it would work on the base response class, but it has some corner-case behaviors which would likely prove confusing. len(IterableResponse(...)) and len(response) when response.data is None are two of the more noteworthy. The first commit in this series defined __len__ on GlobusHTTPResponse, but it has been narrowed to only ArrayResponse.
• some behaviors of __contains__, get() and other methods were untested