Move experimental components that implement run_async to Haystack core

[julian-risch]

We have several components, two document stores and a util function that should be moved to Haystack core / the respective core integration.

• OpenAIChatGenerator . added to haystack core in Add run_async to OpenAIChatGenerator haystack#8880

• InMemoryBM25Retriever added to haystack core in feat: adding async version of InMemoryDocumentStore and associated retrievers haystack#8963

• InMemoryEmbeddingRetriever added to haystack core in feat: adding async version of InMemoryDocumentStore and associated retrievers haystack#8963

• InMemoryDocumentStore added to haystack core in feat: adding async version of InMemoryDocumentStore and associated retrievers haystack#8963

• DocumentWriter added to haystack core in feat: Add async run to DocumentWriter haystack#8962

• utils method added to haystack core in Add run_async to OpenAIChatGenerator haystack#8880

• OpenSearchDocumentStore added to core-integrations in feat: OpenSearch - async support haystack-core-integrations#1414

• OpenSearchBM25Retriever added to core-integrations in feat: OpenSearch - async support haystack-core-integrations#1414

• OpenSearchEmbeddingRetriever added to core-integrations in feat: OpenSearch - async support haystack-core-integrations#1414

Steps for Moving an Experiment to Haystack Core or Integrations

• Make sure the latest Haystack release or an integration release contains the merged experiment
• Update import statements in example cookbook, remove experimental tag from cookbook, etc.
• Close discussion in haystack-experimental with move information
• Remove pydocs
• Remove experiment from catalog in haystack-experimental README.md
• Remove example notebook from haystack-experimental if it exists