admin: making a streaming version of /clusters

/clusters can produce a remarkably huge response. This will need to be fully buffered as an in-memory continuous string. For large configs, this will be slow and may exert memory pressure as well.

See also #31755 as well as prior work streaming admin stats in https://github.com/envoyproxy/envoy/pull/19693. This work is not trivial and has risks. See also the difficult attempt to make /stats/prometheus stream as well: https://github.com/envoyproxy/envoy/pull/24998 which needed to be rolled back due to complexities in the Prom format that made streaming difficult.

See also #32054 which is a similar request for /config_dump