For consistency, renamed both instanceName and storageID to serviceName for consistency.
In all cases, the serviceName is an identifier that differentiates multiple GACAppCheck instances such as when there are multiple FIRApps in use, or multiple dependent SDKs (e.g., Firebase and Google Sign-In). This unique identifier ensures their state is not shared in the Keychain or in User Defaults.
Made the tokenDelegate parameter nullable in the constructor for GACAppCheck since the feature is not used by all integrators. E.g., it is not needed if token refresh isn't used -- 'tokenDidUpdate:instanceName:' would never get called.
- Added parameter documentation to the 'initWithInstanceName:appCheckProvider:settings:tokenDelegate:resourceName:keychainAccessGroup' constructor in GACAppCheck.
- Renamed the umbrella header from AppCheck.h to AppCheckCore.h to match the framework name.
