[Auth] Use native Swift types in Auth backend types

[ncooke3]

Follow-up to #13647 to further attempt to fix #13650.

The crash reports from #13650 show swift_unknownObjectRetain as the last frame in the terminating thread.

Using a symbolic breakpoint, I verified that swift_unknownObjectRetain gets called to get the rpcIssuer used below:

firebase-ios-sdk/FirebaseAuth/Sources/Swift/Backend/AuthBackend.swift

Lines 274 to 275 in b5e2c18

	let (data, error) = await rpcIssuer
	.asyncCallToURL(with: request, body: bodyData, contentType: "application/json")

Stepping through the assembly from swift_unknownObjectRetain, the objc_retain is returned from its source.

When I removed the NSObject conformances and debugged again, swift_unknownObjectRetain is no longer called. I thought this was pretty interesting and will guarantee that should #13650 reappear, the report will at least look different as the object in question will have its memory managed differently as a native Swift type.