List Indexes

Related Issue: https://github.com/hyper63/hyper-adapter-couchdb/issues/32

Admittedly, it's a happy coincidence with CouchDB that allows looking at all of the indexes, since hyper doesn't currently have a `listIndexes` API, but my intuition says we should lock this down and instead introduce an explicit API for listing indexes, with a hyper schema describing its shape, and possible `removeIndex`, once/if the feature requests come in for it.

Start by reading the related CouchDB issue and the repository's existing index behavior and API conventions. Define the shape and semantics of an explicit listIndexes API, including its hyper schema, and establish what completion means; treat removeIndex as a separate future request unless its scope is clarified.

Vom Indexierungsmodell aus dem Issue-Text verfasst.