Add support for Globus Tunnels

[buzztroll]

It includes the API calls needed by programs like the globus-cli.

[kurtmckee]

@buzztroll A changelog fragment is needed. This can be generated using scriv.

pipx install scriv
scriv create
# ...edit, commit, push!

[sirosen]

The main thing we're missing here is tests. We need dummy responses for each of these APIs in src/globus_sdk/testing/data/ and unit tests which run the various methods.
We aren't at perfect coverage today, but we are aiming for 100% on any new work in the SDK.

My overall request is "an initial crack at testing, across the various new methods and new object", not "perfect and exhaustive testing".

I went back through the PR history to find useful reference PRs, and I've got two which should provide decent guidance.

Testing a New API

The most recent new-API was "Update Index" for Search, and looks like a pretty good example: https://github.com/globus/globus-sdk-python/pull/1310/files

Importantly! That PR demonstrates that you don't need to go through a lot of effort to build up some kind of fancy modeling of the responses. Some fixed mocks which have the right shape are totally fine. There are more sophisticated tests with more dynamic responses and whatnot, but we're literally just looking in the base case to make the testsuite run the lines added in src/globus_sdk/services/transfer/client.py.

Testing a New Payload Type

The most recent new-Payload-type was "FlowTimer" for Timers. It has some more complexity in modeling responses, but I really want to point at the test, and the way it "pulls back" the payload after sending it, to examine what was sent:
https://github.com/globus/globus-sdk-python/pull/1311/files

Using

    req = get_last_request()
    sent = json.loads(req.body)

lets us call the relevant method and then assert on what we actually would send to the API.

That might also be a nice way to test the handling of submission_id.

[derek-globus]

Sorry for the slow response on this. Overall I am happy with the direction. A few smaller to think about yet before merging though.

[sirosen]

I just noticed that I had some pending comments which overlap with Derek's. Posting so they are visible.

[sirosen]

Very minor final tweaks needed for us to merge, but I'm approving. I'll merge once we have those last things buttoned up.

[buzztroll]

🎄 🎅 🤶

Removing to unblock the PR