Issue/1953 feat: Add custom operator to Criteria.

[Wuzhengyu97]

Solving #1953 issue
This PR introduces support for custom comparators in the Spring Data Relational library, enhancing the flexibility of query building.

Key Changes

1. Enhanced Comparator
   - Added two new fields:
   - customRenderValueFunc: A function to define custom rendering logic for values.
   - customComparator: A string to define the custom comparator operator.
   - Introduced a setCustomComparator method to configure these properties.
   - Moved the render logic from Criteria to Comparator for better encapsulation and extensibility.
2. Added Custom Comparator Support in CriteriaStep

[Wuzhengyu97]

I realize that my current implementation has problems in concurrent scenarios, and customComparator might lead to data inconsistency. Perhaps using ConcurrentHashMap to store these values would be a better choice. Or do you have a better solution?
@schauder

[Wuzhengyu97]

@schauder @mp911de
HI, guys.
Perhaps I could do something to help move this PR forward. Apart from the concurrency issue, I would like to confirm whether my changes are reasonable.

I have extracted the logic of renderValue into the Comparator. I believe renderValue is strongly correlated with each comparator, so it makes sense to have it managed by the Comparator enum. However, I would love to hear your thoughts on this.

I would greatly appreciate it if you could share your feedback. Thank you in advance!

[schauder]

Why are you closing the PR, if I may ask?

[Wuzhengyu97]

@schauder
There might be some issues with the implementation of the custom operator in this PR. I'll take some time to reconsider and provide a more refined version.