feat: new role for schema discovery

[iFergal]

To be honest, I'm not sure what to call this new role.

It's to address #886. This role is meant to allow a controller to designate a service endpoint (which may be on a different URL than the controller/agent) for discovery of schemas. Perhaps it could be used for other things, which is why I tried to leave it broad as indexer - like an index of OOBIs (or other things, like public ACDCs...) by SAIDs.

Open to suggestions.

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Tested with KERIA/Signify to add a new loc scheme and end role auth where the cid=eid, and the service endpoint appears in the OOBI format http://keria.com/oobi/<cid>.

[daidoji]

@iFergal the platonic role https://en.wikipedia.org/wiki/Platonism or if that word is overloaded in modern english, the Parminedian role after Parmenides. :-)

[2byrds]

@iFergal the platonic role https://en.wikipedia.org/wiki/Platonism or if that word is overloaded in modern english, the Parminedian role after Parmenides. :-)

please no 😆

[daidoji]

zhenxing role https://en.wikipedia.org/wiki/True_form_(Taoism)

[pfeairheller]

We can change the name later if we are inspired by something else.