Update OEP-42 Authentication proposing consolidation of authentication code

Our authentication code is mostly consolidated under edx-drf-extensions and edx-platform's oauth_dispatch (as examples).

Some services use custom authentication code, such as edx-platform XBlocks code or notes custom JWT decoder. These special cases are usually discovered through breaking changes in Production.

This ticket is about codifying our wish to both consolidate authentication code in fewer locations, and to not introduce custom authentication code unless absolutely necessary. Even if the custom code is used in a single service, it is probably better to add it to the consolidated code base, because it almost certainly interacts with the rest of the authentication code.