Ensure additionalTokenRefreshParametersForAuthSession returns a string dictionary

[brnnmrls]

This PR addresses a runtime crash that occurs when the dictionary returned by additionalTokenRefreshParametersForAuthSession: contains non-string values, leading to a type mismatch in GTMAppAuth.

This fix is implemented in GIDEMMSupport and makes the Google Sign-In library more robust by:

• Iterating through the additionalParameters dictionary using the enumerateKeysAndObjectsUsingBlock: instance method
• Checking the type of each value and explicitly converting NSNumber values to NSString
• Safely ignoring any other non-string types to prevent future crashes.

This ensures that the dictionary returned to GTMAppAuth is always of the correct type.

[mdmathias]

I think we want to be a bit more complete than checking for an NSNumber. We can leverage NSJSONSerialization to help us out.

1. Turn the originalParameters dictionary into data via +[NSJSONSerialization dataWithJSONObject:options:error:]
2. Take that NSData and make an NSString from it: -[NSString initWithData:encoding:]
3. Now that you have a string dictionary, make that NSData via [-[NSString dataUsingEncoding:]
4. Pass that NSData to +[NSJSONSerialization JSONObjectWithData:options:error:]

The resulting JSON object should be a NSDictionary<NSString *, NSString *> *. The idea behind step 2 is that it should make everything in the NSData an NSString.

I recommend writing some unit tests with various flavors of types in the values of originalParameters.