Adds auth.rawToken to context to allow access to the underlying token.

[mbleigh]

I've seen this request come through specifically for Genkit, but I think it applies generally -- there are some cases where a user might want to proxy calls to another Firebase service or use e.g. ServerApp to access data as the user. To do this, the original Auth ID token needs to be available to them.

This adds rawToken to AuthData throughout the codebase. If this seems like a good idea I'll go through proper channels to propose the API change.