TransferClientV2

[aaschaer]

Adds TransferClientV2 to experimental, adds its supporting classes, and labels existing v2 support in the TransferClient as Beta as proposed here.

Most of the code in this PR is things for the existing TransferClient that were copied and then slightly modified to work with Transfer v2. Things that are completely new:

• TransferClientV2.list_stream_access_points
• IterableJSONAPIResponse
• JSONAPIPaginator

Some things that aren't included here but would be nice to have at some point:

• some kind of JSON:API document constructor class. This probably can be added without breaking anything as long as it is/acts like a dict.
• better JSON:API support in responses. Not sure if this would be a separate class, or adding convenience attributes/methods to GlobusHTTPResponse that are available if the response looks like a JSON:API response

I also found some issues with the existing tunnels support in the TransferClient while copying things over to TransferClientV2, most notably the iteration on list_tunnels was looking for a v1 "DATA" key. I've included fixes for now, but I'm a bit worried about supporting those going forwards. We likely will make more changes and fixes TransferClientV2 side that might not make it to TransferClient, so maybe we should deprecate those completely rather than leaving them there with a "careful this is Beta" comment?

[aaschaer]

Marking this ready for review as it now has tests and I think its enough to start adding Transfer v2 support to going forwards.

[sirosen]

Minor changes but I think we're quite close to having this finished. 🚀