Guide: working with data-store

[mirceanis]

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[mirceanis]

We need a guide for the @veramo/data-store plugin.
Through that plugin, veramo gains a powerful Query capability over the entities that it manages (Identifiers & keys, credentials, presentations, claims, messages).

Topics to cover:

• setup (different database options)
  • migrations?
  • adapting to existing DBs
• query examples for credentials (of a particular type, having certain claims, having certain issuers, ordering and time constraints)
  • perhaps show off some queries that would be useful for figuring out a Profile
• query examples for Messages

Nice to have:

• some notes on the save parameter that some agent methods use, and how it should be replaced by event driven "save decisions"
• notes on updating existing entries in the DB

Relates to #44 #45 #48