Store trusted authorities for Verifiable Credential issuance

[zonotope]

Store trusted authorities for issuing verifiable credentials granting privileges to query or commit to a ledger within the ledger, and look up these authorities to validate credentials presented in client requests

[mpoffald]

Marking this as blocked on #231 for now, once that lands we can revisit and see what else needs to be done here.

[mpoffald]

@bplatz is this related to your work in #231 ?

[bplatz]

@mpoffald I don't think this is alpha 1 - it would touch the policy work and build on top of it, but brings with it an issue of support for many did methods to be useful - something we don't want in Fluree directly (but would support some method for Fluree to delegate that to an independent service).

This needs to be designed and will end up with I expect multiple tickets here, and a new repo to build a separate service that does this work.