ESQL: Async request disregards format request

[bpintea]

Description

An async request in text mode (POST _query/async?format=txt) will return a JSON body.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[kanoshiou]

👋Hi, I would like to take on the task of addressing this issue using the second method, which involves returning the async parameters as headers. Could I give it a try?

Thank you!

[bpintea]

Issue edited to reflect current behaviour (related, #109576 added HTTP headers with the async details).