stdenvAdapters: add overrideSDK

This is a replacement for using `darwin.apple_sdk_<ver>.callPackage`.
Instead of injecting the required packages, it provides a stdenv adapter
that modifies the derivation’s build inputs to use the requested SDK
versions. This modification extends to any build inputs propagated to it
as well. The `callPackage` approach is not deprecated yet, but it is
expected that it will be eventually.

Note that this is an MVP. It should work with most packages, but it only
handles build inputs and also only handles frameworks. Once more SDKs
are added (after #229210 is merged) and the SDK structure is normalized,
it can be extended to handle any package in the SDK namespace.

Cross-compilation may or may not work. Any cross-related issues can be
addressed after #256590 is merged.

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