List supported providers, or describe one in detail

Without ?id: returns every supported connector with its availability, maturity, category, and sync engine (the connector catalog). With ?id=<provider>: returns what that provider stores and how to use it — indexed_object_types (the streams that become searchable documents), searchable_fields (rendered into the indexed text), filterable_fields (each with the exact filter_key to pass in a query's metadata_filters), the credential_schema for connecting it, and setup_guide (present for providers whose configuration goes beyond the credential schema — e.g. bigquery's per-table cursor/change-history settings and the one-time ALTER statement they may require).