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