Handle `JoinField`

[qcoumes]

• Add JoinField.

• Allow multiple django_opensearch_dsl.documents.Document with different Django model and mapping to point to the same index.

  • Merge the mapping definitions (error on conflict).
  • Store the list of models associated with a doc_class (we currently only store the opposite, i.e. the doc_classes associated with a model).
  • Catch probable errors with an explicit message (e.g. generate_id() needs to be defined for each model)
  • Handle routing since parent and children must be on the same shard (add "_routing": <shard_id> to _prepare_action())

• Update commands to allow filtering on doc_classes in addition to the already existing indices filter.

An attempt has been made on #38, but I’m not fully satisfied with how it was implemented

Resources:

• https://www.elastic.co/guide/en/elasticsearch/client/net-api/7.17/parent-child-relationships.html#parent-child-relationships
• https://www.elastic.co/guide/en/elasticsearch/reference/7.17/parent-join.html