Implement composition/chaining of extensions

[birkland]

This pattern is necessary when multiple extensions are bound to a single resource
If it’s not part of the registry model already, find out how to deterministically arrive at the order in which the request/response are routed through extensions

Depends on:

• Specify extension and service registration API and model #5 (registry model)

Simplifying assumptions:

• Assume that composition/chaining is only a concern when filtering requests to/from repository resources (e.g. it does not apply to exposed URIs)

Testing:

• Verify that order of extensions is deterministic

[ajs6f]

This is something where the SSWAP approach of fully specifying the semantics of services is really nice. You can mechanically infer some notion of validity for a proposed ordering. But that comes at a price, no doubt.