Add FirebaseFunctions module

[darinf]

Changes:

• Updates workflow to a version of firebase-cpp-sdk that includes functions and storage headers and libs.
• Adds incomplete support for mapping between Swift Any and C++ firebase::Variant types -- just what is needed by Arc for now.
• Adds FirebaseFunctionsErrorCode following the pattern for FirestoreErrorCode, et. al.
• Adds C++ type FunctionsRef as a copyable wrapper for the C++ Functions type, so that it can be exposed to Swift.
• Adds HTTPSCallable and HTTPSCallableResult wrapper types.
• Adds basic subset of Functions interface -- just what is needed by Arc for now.

[fiedukow]

Generally looks good, but it's hard to tell just from reading the code statically without tests, if there are no minor typos somewhere. It should be safe for existing functionality though, so we can probably merge and test with arc.

[brianmichel]

This looks so nice and clean, thank you Darin!