Add get_models() and __contains__() in registries.py

[ghkdxofla]

In django-elasticsearch-dsl's registries.py, a public method exists to retrieve a list of registered models.
Also, it can easily check if a particular model is registered by defining the __contains__ method.
The change is to add these two functions.

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (783e89a) 95.65% compared to head (635cca0) 95.34%.

Files	Patch %	Lines
django_opensearch_dsl/registries.py	50.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
- Coverage   95.65%   95.34%   -0.32%     
==========================================
  Files          12       12              
  Lines         576      580       +4     
  Branches       80       80              
==========================================
+ Hits          551      553       +2     
- Misses         21       23       +2     
  Partials        4        4              

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[qcoumes]

Thanks for your contribution !

Can you please add a test for these two methods in tests/tests/test_registries.py?

[qcoumes]

Manually merged in be22b93.

[ghkdxofla]

Manually merged in be22b93.

@qcoumes
Oh... I missed this comment... :(
I checked the commit, and the test code was added just fine!
Next time, I'll add the test code and contribute it as well.