FEAT - Add initial implementation of device code flow

[da1910]

This PR adds support for the device code OAuth2 flow as seen in https://datatracker.ietf.org/doc/html/rfc8628

The basic flow is that a user makes a request to the device_code endpoint which returns a device code, a user code and a verification URL. The user then opens the verification URL in a device of their choice and enters the user code.

The client polls the token endpoint, and once the user has authenticated they will receive a token and optionally a refresh token.

It's a work in progress for initial review as I mean to add subclasses for Okta, EntraID and for Auth0, as well as tidying up the tests and adding a changelog entry.