Investigate search_api_opensearch as base for elasticsearch_connector

Problem/Motivation

Elasticsearch_connector module doesn't have working test coverage and has some dead / Drupal 7 code. The architecture has some fundamental flaws.

Proposed resolution

Investigate https://drupal.org/project/search_api_opensearch as base for elasticsearch_connector. The search_api_opensearch module is rewrite of elasticsearch_connector module with following benefits:
- Synergy effects for two projects using almost the same codebase
- Clear separation of responsibilities for code
- No separate elasticsearch configurations, uses Search API.
- Uses dependency injection wherever possible
- Avoids static methods
- Has a high-level of unit and kernel test code coverage

Remaining tasks

1. Get the updated code
2. Make sure the tests pass
3. Make follow-up issues and link them to this issue
4. Write upgrade paths from 8.x-7.x and add test coverage for this
5. Commit
6. Release — released in 8.0.0-alpha1

User interface changes

1. Remove separate Elasticsearch connector UI.
2. Update the information displayed on the status report (i.e.: hook_requirements())
3. Update the information displayed in the requirements in install.php/update.php (i.e.: hook_requirements())
4. Delete the Elasticsearch-Connector-specific UI — use Search API's UI instead.
5. Delete the help page, as it was not very helpful and very out-of-date.
6. Delete the administer elasticsearch connector, administer elasticsearch cluster, and administer elasticsearch index permissions — use Search API's permissions instead.

API changes

1. Drop composer.json dependency on illuminate/support
2. Drop composer.json dependency on ruflin/elastica
3. Add composer.json dependency on drupal/search_api (and add to elasticsearch_connector.info.yml)
4. Add composer.json dependency on makinacorpus/php-lucene
5. Add composer.json dependency on elasticsearch/elasticsearch
6. Change composer.json development dependencies
7. Delete hook_elasticsearch_connector_load_library_options_alter() hook
8. Delete hook_elasticsearch_connector_supported_data_types_alter() hook
9. Replaced logger.channel.elasticsearch service with logger.channel.elasticsearch_connector and logger.channel.elasticsearch_connector_client
10. Replaced elasticsearch_connector.client_manager service with elasticsearch_connector.cluster_manager service
11. Removed elasticsearch_connector.index_factory service
12. Added services plugin.manager.elasticsearch_connector.connector, plugin.manager.elasticsearch_connector.analyser, elasticsearch_connector.index_param_builder, elasticsearch_connector.query_sort_builder, elasticsearch_connector.query_filter_builder, elasticsearch_connector.search_param_builder, elasticsearch_connector.more_like_this_param_builder, elasticsearch_connector.facet_builder, elasticsearch_connector.query_param_builder, elasticsearch_connector.delete_param_builder, elasticsearch_connector.facet_result_parser, elasticsearch_connector.query_result_parser, elasticsearch_connector.field_mapper, and elasticsearch_connector.synonyms_subscriber
13. Deleted the Elasticsearch Connector Views sub-module (elasticsearch_connector_views) — use Search API's views plugins instead.

Data model changes

1. Remove elasticsearch_connector.backend.plugin.elasticsearch config schema
2. Add plugin.plugin_configuration.search_api_backend.elasticsearch config schema
3. Remove elasticsearch_connector.cluster.* config schema
4. Add plugin.plugin_configuration.elasticsearch_connector.basicauth schema
5. Add plugin.plugin_configuration.elasticsearch_connector.standard schema
6. Remove elasticsearch_connector.index.* schema