FR: Provide an option to set languge for activation links

* Operating System version: Any
* Firebase SDK version: Any
* Library version: 7.0.1
* Firebase Product: auth

Currently it seems like there is no way to set the languge in this metod: **com.google.firebase.auth.AbstractFirebaseAuth#generateEmailVerificationLink**
or in its Async coutnerpart.

The returned url always ends with &lang=en Firebase Auth supports other languages if the end of the url is changed to another language the users land on a page on that given language.
It seems like that the Javascript library has this option already:
[https://firebase.google.com/docs/reference/js/firebase.auth.Auth#languagecode](https://firebase.google.com/docs/reference/js/firebase.auth.Auth#languagecode)

It would be great to be able to set this directly from the library.