Feature: mCSD Explicit Endpoint Accessibility Option

[jlamy]

Is your feature request related to a problem? Please describe.
This is an outcome of the work on the Topologies Whitepaper.

This option allows a directory to support multiple perspectives, by explicitly defining which Endpoint resources are available to be used by particular clients of the directory. Note that this is informational, not a security mechanism; ultimately Endpoints and the resources they serve are protected through security mechanisms such as TLS, PKI, SAML, and OAuth.

Without this option, a community will need to externally specify policy or configuration if it needs to communicate constraints on the use of Endpoint resources in its directory.

Describe the solution you'd like
A client can tell exactly which Endpoints in a directory it is eligible to use. Ideally we would design this in a way where it could be incorporated in a directory not utilizing mCSD in its entirety.

We should make the codes normative that were created in the Topologies whitepaper: HIEInitiator, HIEResponder, PartnerConnectivity, and define an option that makes use of these.

Describe alternatives you've considered
See scratchpad from the Topologies Whitepaper: IHE/ITI.Topologies@cc09b25

Additional context
Add any other context or screenshots about the feature request here.

[lukeaduncan]

@jlamy Now that the whitepaper is done, should we review this again at the next face to face?