Connect to an Elastic Cloud Hosted Deployment acting as a Search API Server by entering a Cloud ID and API key

I've added some code in !91; it needs tests; but reviews are welcome.

We don't have any automated tests for the basicauth, and no specific tests for the standard connector either. Further, search_api_opensearch doesn't have any specific automated tests for its basicauth or standard connectors either.

search_api_solr has test connectors (i.e.: which serve as test doubles in tests), \Drupal\search_api_solr_test\Plugin\SolrConnector\BasicAuthTestSolrCloudConnector, and \Drupal\search_api_solr_test\Plugin\SolrConnector\BasicAuthTestSolrConnector, but the search_api_solr module works in a slightly different way (i.e.: the connectors make the connection to the backend directly; but that's not the case in elasticsearch_connector and search_api_opensearch).

Looking into adding tests for the connectors we have seems non-trivial, so I've created some follow-up issues...

1. ✨ Have ElasticSearchConnectorInterface::getClient() return an Elasticsearch ClientInterface instead of a Client Active
2. ✨ Add tests for the Connectors Postponed

... and I'm going to suggest that we merge this without tests.

Updating the issue summary

I've created a patch from the change link. The change is connecting to an Elastic Cloud instance and working well.

Thanks! I'm glad to hear it is working well!

When someone else (i.e.: who didn't write the code) tests the code, and finds it to be working, we mark the issue as "Reviewed & tested by the community", which I'm going to do now!

We are also using this patch on a project. However, in Elastic Cloud, we are using a serverless project.
This patch was working fine for us, with some extra changes.

In BackendClient, we got errors on the /_close and /_open API calls, so we just had to disable those.
But perhaps this is not the right place and this should be handled in a follow-up issue.