Add CMake logic for defining a future library for playbilling

Adds a cmake script that allows the definition of a version of the future library that uses the namespace playbillingclient instead of firebase. Adds logic to the source files that if playbilling is being used, it includes the generated public header for it instead.
Because of the source libraries needing to include the generated header, it will currently only work for playbilling, but could potentially be generalized in the future.

PiperOrigin-RevId: 251918893

Add a way for users to provide local directories for external libraries

Allow the users to provide their own directories for all the external libraries that are downloaded as part of the build process, by overriding name_SOURCE_DIR and name_BINARY_DIR.

PiperOrigin-RevId: 246938364

Update CMake files with FIRInstanceId's new layout

Instance ID changed their layout to be inline with the other iOS SDKs, so update the CMake scripts to use that layout instead.

PiperOrigin-RevId: 244440380

Change Functions to be more robust if App is deleted out from under t…

…hem.

If App is deleted, invalidate the internal pointers (e.g. FunctionsInternal)
rather than the outer pointers (Functions), so that later code can still
delete the outer pointer.

PiperOrigin-RevId: 238808270