Introduce the MISSING sentinel and use it for HA timeout on GroupPolicies

[sirosen]

The commits here reflect a few quick iterative steps I took to get globus_sdk.MISSING into place and use it.

• Introduce the MISSING singleton: add the singleton and some unit tests, but no usage
• Default to MISSING for GroupPolicies ha timeout: add a first-draft implementation in which exactly one class uses MISSING and is internally responsible for stripping it out before the result is passed to other SDK code
• Automatically strip MISSING from payloads: Refine the implementation to allow MISSING to be passed around, and strip it out when the payload is actually being prepared for use in the transport layer

This final change makes preparation of payload data the responsibility of the PayloadWrapper helper class, and also makes it responsible for recursively transforming any PayloadWrappers to native dicts. Although I was initially concerned that this would introduce a noticeable slowdown, some quick benchmarking revealed this to be very fast on typical data sizes.

We could also push the handling of MISSING "down the stack" into the transport layer, where it would make sense. But for now, it's the client class' responsibility to prune out MISSING.

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In the final state, we should now be able to write client methods like so:

def create_foo(self, x: int | None | MissingType = MISSING):
    body = {}
    body["x"] = x
    return self.post("/foo", body)

Notably, there is not (yet) any support for use of MISSING in query params or other locations. That is intentionally omitted as future work, although it should presumably fit comfortably into this same paradigm.

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📚 Documentation preview 📚: https://globus-sdk-python--885.org.readthedocs.build/en/885/

[kurtmckee]

I've left feedback but this is great as-is.

[kurtmckee]

Oof. This style runs afoul of my similar preference for implicit string concatenation: in parentheses. I'm surprised that black doesn't modify this already, but I may have to adjust to this style.

[sirosen]

It looks like black leaves this in the "user specified style".
I'll change this one -- and BTW I have a much stronger feeling that parenthesized is better here than with the strings.

I'll look into adding a slyp check for this later.

[sirosen]

There's one additional change here, which is to apply "filter MISSING" behavior to query params and headers, along with a test.
It's quite small (last commit right now), but I think it warrants a refresh of the "review clock". 🙂