Add support for Firebase Authentication

[petrpavlik]

Add support for decoding Firebase Authentication JWT tokens.

1. Provide project id when configuring your Vapor application

app.jwt.firebaseAuth.applicationIdentifier = firebaseProjectId

2. Verify the token on a request

let token = try await req.jwt.firebaseAuth.verify()
let email = token.email
let name = token.name
let avatarUrl = token.avatarUrl
// ...

[petrpavlik]

@ptoffy this is the follow up PR to add firebase auth