Add a sub-module to support simple Oauth2 MTM flow

Problem/Motivation

The oauth2_server module supports a lot of complicated flows but has a reported conflict with this module in that both want to use a header of the form "Authorization: Bearer $TOKEN"

A common flow that calls for using a JWT token is the machine-to-machine (MTM) flow with client credentials.

The best practice for MTM is to issue a JWT as the token, so this is something that could be easily handled by this module. The authorization after issuing the token is unchanged since it's simply JWT auth linked to a user account that represents a service account.

Proposed resolution

Add a sub-module the manages client credentials for user account (used as service accounts) and provides an endpoint for issuing a short lived (e.g. 1 hour) JWT in response to client credentials.

Remaining tasks

Implement, add tests

User interface changes

New tab on user profiles (and controlling permission) to add/manage client secrets. Like the users_jwt module, This will include the option to generate the client secret.

API changes

n/a

Data model changes

n/a

Will use user data storage service like users_jwt module