The arm supplying an externally-implemented backend.
adapter is required by the union, so a 'custom' registration that
forgets it is a compile error rather than a startup throw.
readonly
type: "custom"
Selects the external-adapter arm.
Since 0.2.0
readonly
adapter: IDatabaseAdapter
The backend to use, already constructed. The plugin calls connect() on
it during register() and disconnect() during shutdown; it never
constructs or replaces it.