Add support for secret management and from_env_var for OpenSearch integration

[aantti]

When using OpenSearch as document store it's not currently possible to utilize secret management and easily pick credentials for HTTP basic auth from environment.

It can definitely be instrumented along the lines of:

    return OpenSearchDocumentStore(
        hosts=settings.opensearch_host,
        http_auth=(settings.opensearch_user, settings.opensearch_password),

while managing settings separately.

However, there's also a somewhat bigger problem when trying to use YAML to override pipeline architecture (or just load pipeline definitions from YAML), because it would currently look like:

  document_writer:
    init_parameters:
      document_store:
        init_parameters:
          create_index: true
          embedding_dim: 768
          hosts: 
            - https://localhost:9200
          http_auth:
            - admin
            - your-password

which isn't ideal for any serious deployment scenarios.

If would be great to have an option to use secret management and consequently env_vars supported for YAML for OpenSearch.