[FR] auth useEmulator normalize definition

The definition of `useEmulator` function in most of packages (`firestore`, `functions`, ...) use 2 params : `host` and `port`

But in the package `auth` the function `useEmulator` take only 1 param : `url`

I don't know if there is a reason about these differences If there isn't, it would be interesting to normalize this with same definition for all package : `useEmulator(host, port)`

Compare the useEmulator definitions in the auth package with those in firestore and functions, starting from the package API declarations and existing emulator tests. Confirm whether auth has a reason to accept only a URL; if not, normalize the public signature and update its tests so host-and-port usage is covered.

Written by the indexing model from the issue text.