Add IRIs for mappings

[marrus-sh]

Right now mappings are defined like :—

mappings:
    oai_dc:
        name: "OAI/DC"

I would like to add an iri (or uri) property in addition to name for mappings.

Motivation

Our API supports multiple mappings via a profile media type parameter, which must be an IRI. For example, we use the following Accept: header to request OAI/DC content:

Accept: application/ld+json; profile="tag:surfliner.gitlab.io,2022:api/oai_dc"

It would be nice if Houndstooth supported looking up metadata mappings by IRI, so that we wouldn’t have to make assumptions in our application about which mapping tag:surfliner.gitlab.io,2022:api/oai_dc refers to.