Add support for OpenAPI parameter schema types

[robertcv]

My current setup:

Django==2.2.14
djangorestframework==3.10.3
django-filter==2.3.0

I am working on documenting my API and found that DRF can use OpenAPI schemas to automatically generate documentation.

I noticed that all parameters schema type gets set to string although I am using non-string filters like for example the BooleanFilter. OpenAPI seems to support other schema types and formats. Upon further investigating I found that DRF is calling DjangoFilterBackend.get_schema_operation_parameters to get schema parameters and this intern always sets the type to string regardless of filter type.

My solution was to override the beforementioned function and set appropriate types and formats but this functionality should probably be part of django-filter. Perhaps every Filter should define schema type/format as class parameters which are then used when generating the schema.

[carltongibson]

Moving to #1432