Better abstraction around hosted databases (SQLite, DuckDB, etc.)

[marcua]

• The hosted_db module doesn't abstract away database functionality much, and breaks the abstraction in a few places
  • In the main hosted_db.rs, there is a run_query that matches on db_type. We only support one DB type for now (SQLite), and so this function throws an error for any other db_type.
  • The SQLite-specific code does a mix of handling sandboxing (something that shouldn't be DB-specific) and SQLite-specific querying
  • Sandboxing assumed you're running queries on SQLite with no abstraction
• Outside of the querying logic, there's also server::snapshots::execution, which calls directly into the SQLite backup facilities. Ideally that would also be in some form of abstraction.
• All of this adds up to
  • Create a HostedDb trait (like ayb_db::db_interfaces::AybDb)
  • Specific implementations like SqliteHostedDb and DuckDbHostedDb would then implement query (with safe/unsafe variants) and backup
  • Isolation should be orthogonal to the HostedDb. Perhaps the query code in the HostedDb can pass whatever information it needs to an isolated context to run inside of it.