SPIKE: investigate authentication performance optimizations

We are expecting that the authentication overhead (the time it takes for Okapi to call mod-authtoken during the "auth" filter phase for /*) is substantial. If this is confirmed by OKAPI-860 and PTF measurements we need to investigate potential optimisations to minimize this overhead.

In the past we have (roughly) discussed different potential approaches:

• cache and re-use authentication tokens for module-to-module calls (for the same endpoint-modulePermissions pair)
• merge mod-authtoken with Okapi (as an internal module) (TODO link the Okapi caching ticket)
• modify internal API between Okapi and mod-authtoken to be able to provide more caching within mod-authtoken (e.g make mod-authtoken responsible for retrieving module permissions (or complete MDs) and cache tokens localy)

We would like to introduce the optimization in a way that is as transparent to FOLIO modules as possible and does not require fundamental changes to the authentication/authorization model. We may need to change certain aspects of existing behavior, e.g the fact that module-to-module communication "inherits" initial user permissions, to benefit from more aggressive token caching and re-use.