Investigate if `GCMSenderID` is still a required parameter.

[ryanwilson]

Feature proposal

• Firebase Component: Core

Investigate if gcmSenderID is still a required parameter for FirebaseOptions. It's currently in the default initializer but may not be needed after IID is no longer a requirement.

If removed, we'll need to evaluate changes needed for IID/Messaging in order to deal with a missing gcmSenderID.

[ryanwilson]

Related to #4692

[paulb777]

There are references in IIID, FIS, Messaging, FIAM, RC, and Segmentation

[ryanwilson]

Should likely make this optional since users can use Firebase without the above mentioned products.

[ryanwilson]

Moved ahead to the 8.14.0 milestone - we'll need to deprecate it if needed.