Make it possible to send headers to Elasticsearch clients from config

In pursuit of https://github.com/elastic/enterprise-search-team/issues/3553

From SDH https://github.com/elastic/sdh-enterprise-search/issues/1022

On cloud, we now need to pass-through the x-elastic-app-auth header.

This PR is a reflection of Python PR https://github.com/elastic/connectors-python/pull/243 and just makes it possible to specify elasticsearch.headers in config file. These settings are then passed to elasticsearch client when it's initialised.

Checklists

Pre-Review Checklist

[x] Covered the changes with automated tests
[x] Tested the changes locally

Changes Requiring Extra Attention

[ ] Security-related changes (encryption, TLS, SSRF, etc)
[ ] New external service dependencies added.

Related Pull Requests

[<!--List any relevant PRs here or remove the section if this is a standalone PR.

https://github.com/elastic/.../pull/123-->](https://github.com/elastic/connectors-python/pull/243)

For Elastic Internal Use Only

[ ] Considered corresponding documentation changes to contribute separately
[ ] New configuration settings added in this PR follow the official guidelines
[ ] Built gems (both connectors_utility and connectors_service) and included into Enterprise Search and tested that Enterprise Search works well with new gem versions. Instruction can be found here

Status	Branch	Result
✅	8.5	https://github.com/elastic/connectors-ruby/pull/503
✅	8.6	https://github.com/elastic/connectors-ruby/pull/504

elastic / connectors-ruby

Make it possible to send headers to Elasticsearch clients from config #502