feat(filters): add `OperatorType.custom` for custom backend service

[ghiscoding]

• adding a OperatorType.custom so that the users can use BUT the developers will have to implement their own code to go with it. This operator does nothing by itself unless the developers implement some code to go with it. It was only added to provide an easy way for the users to choose a valid operator.
• typically the developers will have to provide his own code implementation, typically a backend service (it could be by extending and overriding the OData/GraphQL service with their own usage).
• it was added because of this Satck Overflow question

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.8%. Comparing base (9171b96) to head (7279812).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1526 +/- ## ======================================== + Coverage 99.8% 99.8% +0.1% ======================================== Files 198 198 Lines 21616 21617 +1 Branches 7219 6949 -270 ======================================== + Hits 21555 21556 +1 - Misses 55 61 +6 + Partials 6 0 -6 ```

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