set concept_id as a path parameter instead of a query parameter

It would be more consistent with applications like `titiler-pgstac` and the OGC APIs if we used `concept_id` as a path parameter rather than a query parameter.

Instead of `{titiler_cmr_endpoint}/statistics?concept_id=C12345-abcdef` it would be `{titiler_cmr_endpoint}/collections/{concept_id}/statistics`