support being the backend for OpenSearch Dashboards

endpoints opensearch dashboards uses:

• GET /_nodes => support getting some information about cluster. Could be mocked for a MVP
• GET /_alias/<name> => report whether an alias exists
• PUT /<name> => create index, we need this for .kibana_1, but also for opensearch_dashboards_sample_data_logs&co (loading sample data, which is for all intent and purpose a normal index). for .kibana, it uses features we don't support (nested fields at least)
• POST /_aliases => create an alias (from .kibana to .kibana_1 mostly)
• * /.kibana* => these indexes will need to be handled out of the normal index flow: we need to support document update & co
  • GET _doc/<doc_id> => return doc by id
  • DELETE _doc/<doc_id> => delete doc by id
  • PUT _create/<doc_id>?refresh=wait_for => create doc with id
  • POST _update/<doc_id>?refresh=wait_for&_source_includes=.* => update existing doc
  • POST _search => normal search (on a not so normal index), some queries use nested fields :-/
  • GET _refresh => probably a no-op (every call will be sequentially consistent)
  • GET count => return number of docs in index
• ANY /_plugins/.* => i hope we can mock/not implement these. osd seems fine with at least some returning 400.
• POST /_bulk => in most case unchanged, but we need to support writing to .kibana (we may want to forbid writing in kibana and anything else)
• HEAD/GET /<name> => verify index exists
• POST /_mget => get multiple docs by id (only for .kibana)
• GET /_cat/templates/* => report no template exists
• GET /_cat/plugins?* => report no plugin exists?
• GET /_resolve/index/* => already exists, plug .kibana indexes inside

note: all sample have geo data we don't support, we should see to fail as gracefully as possible, it would be nice for everything else to keep working

Start from the existing /_resolve/index/* handling and the normal index flow, then trace how the listed OpenSearch Dashboards endpoints are routed. Implementing this requires deciding how .kibana*, aliases, bulk operations, plugins, and unsupported geo data should behave. Done means the listed dashboard and sample-data operations work or fail gracefully.