Entitlements Endpoint: Update to align with ADR 472

[ttschampel]

Update Entitlements Endpoint to Align with ADR #472

The rough task list:

• Define entity resolution service interface (ERS)
• Move Keycloak IDP Plugin to a service implementing the ERS contract
• Move the JQ Custom Built-In Function to a Subject Mapping Built-In Function resolve encapsulating both the JQ Selector and condition evaluation currently residing in Rego.
• Update the platform to use a single Core Rego file that consists of fetching the entity representation for a configured ERS endpoint and then passing to the new Subject Mapping resolve function
• Remove Keycloak specific inputs in OPA engine call.

[jrschumacher]

@jakedoublev anything we should consider for policy with regards to acceptance criteria?

[elizabethhealy]

i'm getting started on the third bullet Move the JQ Custom Built-In Function to a Subject Mapping Built-In Function resolve encapsulating both the JQ Selector and condition evaluation currently residing in Rego.