Extension Mechanism (aka plugins)

[alexsnaps]

See this draft RFC for more details...

This is the master issue with regards to the implementation as a private API initially, using it to support implementing #1148 & #1149

• Out-of-process plugin management;
• Define & implement the gRPC API with a way to resolve "core CRs" derived from "meta" ones (concretely here: a way for the PlanPolicy to identify the AuthPolicy underlying the targeted OpenIDConnectPolicy;
• Out-of-process ready probe;
• Out-of-process liveness probe & restart;
• <Specialized>IdentityPolicy example #1148
• PlanPolicy exploration #1149