Create IDP interface and "plug in" keycloak

[pbacon-blaber]

We need to update the keycloak uses in the Entity Resolution Service to be a "plug in" to a more generic IDP interface. More details can be found in the Interface Design section of the Confluence doc that was created as a result of #420.

Acceptance Criteria

• New interface for interacting with IDPs created as detailed in the linked confluence document above
• Keycloak implementation of that interface created and used
• All of the existing tests around keycloak should still be working with the plugin without any major changes
• Create new tests around the interface