feat/rm auth

[siddhantk232]

remove auth and related features

Authentication related feature and routes are removed, specifically:

• auth handlers for email_password and github OAuth providers which were
  working

• other untested OAuth providers (discord, twitter, telegram)

• Below listed processor yield an error when used, these used to provide "mixed
  privacy" features which was never full implemented nor documented anywhere.
  We will bring a revised version "mixed privacy" feature in future.

  • document-readers
  • user-groups
  • user-group-by-id
  • get-identities
  • is-reader

• http::Request::ud is marked todo!(): This is a major breaking change as
  we do use it in our internal codebase. There are three ways I can think of by
  which we can keep this function in fastn-core:

  1. we use fastn-community/auth as a dependency and use a function exposed by its rust
     code in here
  2. we use tokio-postgres and assume fastn_user table exists when FASTN_ENABLE_AUTH is
     set. ud() will also only work when FASTN_ENABLE_AUTH is set
  3. we suggest everyone to use fastn-community/auth as a dependency and use
     this function from there.

  I think we should either go with first or third option. Going with third
  means that fastn does not know or care about auth at all which imo is a good
  thing.

• remove diesel dep

[amitu]

I agree with the third option. Let's try to use auth stuff at FifthTry the way we want our end customers to use it.