[Auth] Convert *Response classes to structs

[ncooke3]

I want to experiment with an alternative approach to #14006 where AuthBackend is instead made an actor. One of the first errors I hit when doing so is that the T.Response types need to be Sendable. Making them structs satisfies that constraint. I'm breaking this up into it's own PR as it's a prereq to exploring an alternative approach to #14006 and will be needed in either approach.

One important consideration when doing this is to ensure nothing depends on these new structures having reference semantics, else they may be left referencing an outdated copy. I audited all usages of AuthBackend.call(request: Request) -> Request.Response and the returned response was always a locally captured let, so if the response would have been mutated, the response would have been assigned to a var. Additionally, I would have expected to see something like: self.savedResponse = response, etc.

#no-changelog

[andrewheard]

LGTM. In a follow-up PR, let's investigate if https://github.com/firebase/firebase-ios-sdk/pull/14012/files#diff-870dc0117add1bfe1fac98e8c6559777840cef238c3c14f22c9a2c0dcfa356f0R254-R308 can be reasonably refactored to set the fields at creation time to avoid the need for AuthRPCResponse.setFields to be mutating.