[FR] Verify password server-side

**Is your feature request related to a problem? Please describe.**
I want to create a GraphQL API through which everything server-side is handled, including email-password authentication. Also, I want to make use of Firebase Authentication. I am able to register users with their email and password server-side, however, I am not able to verify their password when they are logging in through the GraphQL API, since the auth module of the node admin sdk does not support 'signing in', or checking the password in any other way server-side. I know I am able to recreate such a use case by letting the user login with the client side sdk, and after that, sending an id token to authenticate themselves, but that would require the client to have firebase installed, which is something I'd like to avoid.

**Describe the solution you'd like**
Something like `const userRecord = await auth().getUserByEmailAndPassword(email, password)` or `const isPasswordValid = await user.verifyPassword(password)`

**Describe alternatives you've considered**
Using an ID token sent from the client, but that would require you to install firebase on the client.