ESQL streaming PoC

[lukasolson]

Summary

Related: #191003

Really rough PoC on top of elastic/elasticsearch#111840:

• Only works with bfetch disabled (bfetch:disable advanced setting set to true)
• Adds an artificial delay to ES|QL requests to show this works for both submitting & polling on a _query/async request
• Does not parse ES response server-side, but forwards the Readable stream from the ES JS client to the HTTP response
• Reads ID (x-elasticsearch-async-id) and completion status (x-elasticsearch-async-is-running) from headers so that parsing the body can happen on the client rather than both on the client & server

[lukasolson]

Closing in favor of #193060.