Allow ElasticsearchEmbeddings to create a connection with ES Client object

[jeffvestal]

This PR adds a new method from_es_connection to the ElasticsearchEmbeddings class allowing users to use Elasticsearch clusters outside of Elastic Cloud.

Users can create an Elasticsearch Client object and pass that to the new function. The returned object is identical to the one returned by calling from_credentials

# Create Elasticsearch connection
es_connection = Elasticsearch(
    hosts=['https://es_cluster_url:port'], 
    basic_auth=('user', 'password')
)

# Instantiate ElasticsearchEmbeddings using es_connection
embeddings = ElasticsearchEmbeddings.from_es_connection(
  model_id,
  es_connection,
)

I also added examples to the elasticsearch jupyter notebook

Fixes # https://github.com/hwchase17/langchain/issues/5239

cc: @hwchase17