[Functions] Complete Swift 6 support

[ncooke3]

This PR gets Function to complete Swift 6 support.

Note: I did add a TODO in the Functions error handling. In theory, it should be okay, but in practice, it will be hard to get checked Sendable until FunctionsSerializer no longer returns Any. This appears feasible but it will take some more investigation and I'd prefer to circle back to it to focus on others areas of Swift 6 compliance.

Reasoning for changes in FirebaseFunctions/Sources/HTTPSCallable.swift:

• The sending param attribute does not bridge to ObjC. Even passing an immutable type from ObjC crashed the integration tests. To workaround, I'm marking the existing signature ObjC-only and adding a new Swift-only function with the sending param. It should otherwise match the existing signature and not be a break to Swift 5 clients because Sendability changes are non-breaking. The ObjC-only calls through to it with a SendableWrapper type that seems to appease the runtime.

#no-changelog

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