Experimental support for Azure managed service identities

[richardbarrell-calvium]

Related to #12562, I happen to need support for Azure managed service identities.

I believe that the feature which #12562 is asking for should be extremely similar to this, because that also involves sending a HTTP request to a local endpoint and then creating a token from that.

Please note TODOs in quaint/src/connector/mssql/native/mod.rs:

• I'm not certain that this is the right place to fix this problem, maybe I should have put the change in tiberius instead?
• these tokens do expire; currently I have no mechanism to refresh them

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[richardbarrell-calvium]

Just thinking aloud here:

1. Maybe a better implementation would be to use https://crates.io/crates/azure_identity/0.19.0/dependencies instead of raw reqwest to construct these requests and headers.
2. I'm not sure how to make a unit test work for this feature (i.e. against a locally running copy of SQL server in a docker container) because this relies on a bunch of MS Entra (recently renamed from Azure Active Directory) stuff.