Implement MGLForegroundStyleLayer.sourceIdentifier

[frederoni]

Fixes #7557

This PR adds a sourceIdentifier getter for all concrete subclasses of MGLForegroundStyleLayer.

[mention-bot]

@frederoni, thanks for your PR! By analyzing this pull request, we identified @1ec5, @jfirebaugh and @boundsj to be potential reviewers.

[jfirebaugh]

#7557 claims that the underlying core classes contain getters and setters for source ID, but in fact setters did not exist and are being added in this PR. Changing the source ID is an operation that's more complex than simply updating the data member in the impl. Beyond that, the source tiles will need to be reparsed, and there are places in core where layers are grouped by source that will need to be updated. We'll need to add test cases to the test-suite, and port this feature to GL JS. Overall, adding a setter may turn out to be a rather complex change. Are we sure it's necessary, especially at such a late date in the release cycle?

[frederoni]

I can see some convenient use cases for it but it's not an absolute necessity.
Let's drop the changes in core and keep the getter in order to fix the crash in #7557

[1ec5]

My mistake: we should make the SDK property read-only. Sorry for the confusion.