Create usage logs (invocations) for all entities with secrets

[mike-marcacci]

As a prerequisite to #49 we need the ability to track when various entities with secrets are used. While this is currently possible via external logging systems, this kind of invocation log (distinct from an access log) should be a first-class citizen. Adding core support will enable this information to be exposed over the GraphQL API, and allow AuthX to enforce granular access control to it.

A credential is invoked:

• when it is used for authentication

An authorization is invoked:

• when a bearer token is generated
• when a basic token is used to access AuthX

A grant is invoked:

• when it is exchanged for an authorization by its client

A client is invoked:

• when it exchanges a grant for an authorization