Execute dense searches in parallel in OpenSearchDocumentStore when in batch mode

[tstadel]

Is your feature request related to a problem? Please describe.
If we call run_batch OpenSearchDocumentStore makes use of msearch if it is a sparse query. That is, it executes the searches in parallel. For dense searches this is not the case, all searches are executed in sequence.

Describe the solution you'd like
OpenSearch's msearch supports dense searches as well, so let's:

• create a batch pendant of query_by_embedding like query_batch for DocumentStores
• default impl would be to delegate to query_by_embedding
• for OpenSearchDocumentStore and ElasticsearchDocumentStore implement it to use msearch

Describe alternatives you've considered

Additional context
Parallel searches using msearch substantially speed up batch queries: I measured ~50% latency drop.