Allow defining of select_related per include

[Anton-Shutik]

Fixes #591

Description of the Change

This PR adds SelectAndPrefetchForIncludesMixin which handles include GET parameter and adds select_related and/or prefetch_related calls to queryset in order to save some sql queries.

This also deprecates PrefetchForIncludesHelperMixin.

AutoPrefetchMixin is modified so if developer declared prefetch_for_includes that means that it's handled "manually" and no need to auto prefetch related entities.

Let's say we have a code like below:

class Product(models.Model):
    name = CharField()
    tags = M2MField()
    variant = FKField()


class ProductSerializer():
    tags = ResourceRelatedField()
    

class VariantSerializer():
    product = ResourceRelatedField()


class VariantViewset():
    serializer_class = VariantSerializer
    prefetch_for_include = {
    'product': 'product__tags'
}
    

Line 'product': 'product__tags' will save one query per included product, because it will prefetch tags.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[sliverc]

Thanks for working on this. See my inline comments. And I assume once we have settled on the implementation you will follow up on the other PR TODOs.

[codecov]

Codecov Report

Merging #600 into master will decrease coverage by 0.27%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #600      +/-   ##
==========================================
- Coverage   95.66%   95.39%   -0.28%     
==========================================
  Files          56       56              
  Lines        2861     2865       +4     
==========================================
- Hits         2737     2733       -4     
- Misses        124      132       +8

Impacted Files	Coverage Δ
example/tests/test_performance.py	100% <100%> (ø)	⬆️
rest_framework_json_api/compat.py	50% <0%> (-50%)	⬇️
rest_framework_json_api/django_filters/backends.py	86.66% <0%> (-13.34%)	⬇️

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[codecov]

Codecov Report

Merging #600 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #600      +/-   ##
==========================================
+ Coverage   96.16%   96.16%   +<.01%     
==========================================
  Files          56       56              
  Lines        2866     2870       +4     
==========================================
+ Hits         2756     2760       +4     
  Misses        110      110

Impacted Files	Coverage Δ
example/tests/test_performance.py	100% <100%> (ø)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[sliverc]

This PR is shaping up nicely. I like the auto select_related part.

See my inline comments for some last changes and questions.