test: Enhance and formalize keycloak-powered local IdP capabilities

John Nagro originally leveraged Keycloak to provide an identity provider
for testing SAML-based third party auth (TPA) in edx devstack, but it
was not broadly usable because it lacked scripts for keycloak
provisioning, LMS TPA provisioning, and other necessary scaffolding to
tie it all together.

ENT-11566