Legacy Google sign-in library will be deprecated

[J-avery32]

We use the legacy JS google sign in library. This should be updated.

[dylanmcreynolds]

I would really like to remove the javascript google stuff altogether. I'd rather handle the OIDC exchange ourselves. Here's a brief look at the flow:

I realize that much of this is work in splash-server, but let's put all the details here.

Much of what I'm talking about can be seen in https://github.com/AI-multimodal/oidc_validator. That repo is a stripped out project that I created to test dealing with OIDC just on its own. Some of that was used to make Tiled play with OIDC.

I think we have a configurable model for "OAuth2IdP" in he server. It's something along the lines of:

class OAuth2IdP:
  authURL: string # location that the user is taken to to perform logni. e.g. https://accounts.google.com/o/oauth2/v2/auth
  callbackURL: string # splash-server location that the IdP redirects to in order to exchange codes
  successURL: string # splash-server local location to take the user to after successful login
  failureURL: string # splash-server local location to take the user to after failed login
  IdPClientId: string # client id to send to the IdP
  IdPClientSecret: string # client secret to send to the IdP
  scopes: list[string] :  a list of scopes to send to the provider