Extract token functionality from Oauth2DrupalAuthProvider into a service

Problem/Motivation

I wanted to add an Access Check handler to allow _oauth_scope to be added on route requirements. However, there was no easy way to get the scopes from a request without re-implementing part of the OAuth2DrupalAuthProvider service.

Since that service is very specialised, it makes sense to extract the useful parts of that method into a separate service and expose that instead.

Steps to reproduce

N.a.

Proposed resolution

Implement an OAuth2Helper service that contains methods to retrieve the token and scopes from a request.

Remaining tasks

Review

User interface changes

None

API changes

The OAuth2DrupalAuthProvider no longer contains the protected getInfoToken method.

A new oauth2_server.oauth_helper service exists that allows you to easily fetch OAuth2 related data from a request.

Data model changes

None