Improve Requestish type

[tomgrin10]

Currently certain type checkers don't accept usage of fastapi's Request as an argument for authenticate_request because of a type discrepancy.
The error I'm getting with basedpyright is this:

Argument of type "Request" cannot be assigned to parameter "request" of type "Requestish" in function "authenticate_request"
  "Request" is incompatible with protocol "Requestish"
    "headers" is invariant because it is mutable
    "headers" is an incompatible type
      "property" is not assignable to "Dict[str, str]"

I changed the Requestish type to be more correct to its usage.
There are no checker linter warnings now.