feat: Add Couchbase as an online store

[ejscribner]

What this PR does / why we need it:

Adds the ability to use Couchbase as an online store.

Which issue(s) this PR fixes:

N/A

Misc

While the docs focus on using this Online Store with Couchbase Capella Operational (Couchbase's cloud offering), the tests have all been set up to run with Couchbase Server. The subsequent Offline Store that will follow will require Couchbase Capella, hence the emphasis in the docs.