feat: Add JoinField and update indexation command to select on which …

[Virgin75]

…models to start the indexation

• Add JoinField in fields.py to allow easy creation of parent/child join type field.
• Edit indexation management command to allow start of the indexation on specific models only (with --objects).
  Usage, e.g: python3 manage.py opensearch document index --objects stay document will only start the indexation for the registered models using the Stay and Document models (defined in Django inner class).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 43.75000% with 36 lines in your changes are missing coverage. Please review.

Project coverage is 90.00%. Comparing base (4d4f437) to head (8980d89).
Report is 5 commits behind head on master.

❗ Current head 8980d89 differs from pull request most recent head 4f04a28. Consider uploading reports for the commit 4f04a28 to get more accurate results

Files	Patch %	Lines
...o_opensearch_dsl/management/commands/opensearch.py	42.10%	30 Missing and 3 partials ⚠️
django_opensearch_dsl/registries.py	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
- Coverage   95.93%   90.00%   -5.94%     
==========================================
  Files          12       12              
  Lines         615      620       +5     
  Branches       86       93       +7     
==========================================
- Hits          590      558      -32     
- Misses         20       55      +35     
- Partials        5        7       +2     

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