This PR fixes an issue introduced by #402 related to AppAuth's use of a nonce, fixes an issue in GSI's -[GIDSignIn maybeFetchToken:], and provides an example in the DaysUntilBirthday app.

Issue in #402

#402 provides the ability to pass a manual nonce via GSI to AppAuth (e.g., via this OIDAuthorizationRequest initializer that takes a nonce parameter).

Unfortunately, this addition to GSI breaks AppAuth's automatic use of its own nonce if one is not supplied since #402 will always pass through -[GIDSignInOptions nonce]. So, if the nonce in GIDSignInOptions is nil, then nil will be passed to AppAuth for the nonce, and therefore AppAuth will not produce its own nonce via this initializer.

Fixing -[GIDSignIn maybeFetchToken:]

-maybeFetchToken previously used a method on AppAuth (+[OIDAuthorizationService performTokenRequest:callback:])
that would pass nil for the last authorization response. The consequence of this is that the last authorization response's nonce will not be available for comparison within AppAuth since the last auth response and its nonce would be nil in the comparison.

This PR fixes the issue by always passing along the last authorization response in -[GIDSignIn maybeFetchToken:].

DaysUntilBirthdayExample

This PR also provides an example of the client passing in a manual nonce, and then comparing the received nonce in the authorization response to the manual nonce. It provides a simple example that clients may use to develop their own solution for comparing their manual nonce to the authorization's returned nonce