Gets a single trigger source with full provider_data.

For pipedream sources, provider_data is the Pipedream component
descriptor; for enriched Pipedream sources the response also includes
an import_properties_request.

Native sources also include top-level transport metadata:
delivery_modes, preferred_delivery_mode, and transport_capabilities.

For native sources, provider_data carries:

• fetch_skill_id — UUID of the synced trigger skill.
• polling_skill_id — temporary deprecated alias for fetch_skill_id.
• arg_specs — the Annotated[...] argument specs of the trigger skill.
• category — trigger.

The response id is "<provider>:<provider_key>" and can be
round-tripped through the parser used by triggers wiring code.