feat: add public Refdb type

[bordumb]

Overview

This is a followup PR to this one:
#1221

Specifically addressing the comment from @ehuss here:
#1221 (review)

2 additions:

• The aim is to add a common interface for interacting with the reference database, introducing a dedicated Refdb type rather than exposing refdb operations solely as convenience methods on Repository
• Add 1 additional feature set_refdb() as an example of how this new interface make extending it easier

1 change:

• Renamed Refdb to RefdbMarker to avoid confusion with the new public Refdb type in refdb.rs; this is just a lifetime marker, not a user-facing type