Filtering interface implementation

[korycins]

Backend part - #2588

This feature introduces base filter logic that can be used in any graphQL queries.

How it works:
I've created EnumFilter which handle graphQL enums, it was needed to serve enum fields over api. Based on django-filters and EnumFilter we create new FilterSet object with all fields that we want to use in filters. Our custom FilterSet should be assigned in a derived class from FilterInputObjectType. FilterInputObjectType is responsible for converting all django-filter's fields to graphene fields (also enum fields). An object of a class that derives from FilterInputObjectType should be assigned as a new param in FilterInputConnectionField (by default it should be filters=FilterInputObjectType(), but you can also pass custom filters name - All filters will be available under this name in API).
You can also check the test: test_filter_input which. The test contains configuration step by step.

What do you think? If you like it we can think about the implementation of the filters for the rest of the queries. For now, I've added some filters only for products view.
We can also think about some small documentation chapter about filtration.

Pull Request Checklist

1. Privileged views and APIs are guarded by proper permission checks.
2. All visible strings are translated with proper context.
3. All data-formatting is locale-aware (dates, numbers, and so on).
4. Database queries are optimized and the number of queries is constant.
5. Database migration files are up to date.
6. The changes are tested.
7. The code is documented (docstrings, project documentation).
8. GraphQL schema and type definitions are up to date.
9. Changes are mentioned in the changelog.

[NyanKiyoshi]

Whenever you have 5 minutes before switching from draft to ready for review, could you add look back at the changes and put some more comments so the code will be quicker to read when not familiar with the implementation?

[korycins]

@NyanKiyoshi No worries :)

[codecov]

Codecov Report

Merging #3952 into master will decrease coverage by 0.02%.
The diff coverage is 90.86%.

@@            Coverage Diff             @@
##           master    #3952      +/-   ##
==========================================
- Coverage    91.3%   91.27%   -0.03%     
==========================================
  Files         267      275       +8     
  Lines       14501    14953     +452     
  Branches     1393     1448      +55     
==========================================
+ Hits        13240    13649     +409     
- Misses        894      913      +19     
- Partials      367      391      +24

Impacted Files	Coverage Δ
saleor/graphql/product/resolvers.py	94.56% <100%> (+0.75%)	⬆️
saleor/graphql/discount/schema.py	100% <100%> (ø)	⬆️
saleor/graphql/order/schema.py	100% <100%> (ø)	⬆️
saleor/graphql/core/types/__init__.py	100% <100%> (ø)	⬆️
saleor/graphql/product/enums.py	90.47% <100%> (+2.59%)	⬆️
saleor/graphql/core/types/converter.py	100% <100%> (ø)
saleor/graphql/product/types/attributes.py	100% <100%> (ø)
saleor/graphql/account/schema.py	100% <100%> (ø)	⬆️
saleor/graphql/core/types/common.py	100% <100%> (ø)	⬆️
saleor/graphql/order/types.py	83.24% <100%> (+0.08%)	⬆️
... and 26 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d0ba739...84881bb. Read the comment docs.

[NyanKiyoshi]

LGTM, good job! 🎉

[maarcingebala]

@korycins Looks good! I have few notes to change later but I'd really like to merge it as it is and make some improvements later. Remember to update the changelog ;)