Only return certain fields

Hello,

I was wondering if you could add functionality to only return certain fields?
I'm loading in thousands of messages and it would save memory to only return the body and header when a user clicks a message.

References: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-fields.html

Kind regards,
Marten Mooij

Start by tracing how esquery builds Elasticsearch search requests and compare its current output with the referenced Elasticsearch Search fields documentation. Clarify the API for selecting fields such as the message body and header, then add coverage to the project's existing query tests and verify the generated request.

Written by the indexing model from the issue text.