Add support for filtering by ECK entity IDs in Views Reference Filter

Problem/Motivation

Currently, the Views Reference Filter module does not support referencing entity IDs created via the Entity Construction Kit (ECK).
This limits the module's flexibility when working with custom entity types, as it's not possible to filter Views based on ECK entity references.

This functionality is especially useful in cases where content needs to be dynamically filtered by relationships to custom entities,
such as filtering nodes or users based on ECK-managed references.

Steps to reproduce

1. Create a custom entity type using ECK.
2. Create a View that displays content referencing the ECK entity.
3. Attempt to add a Views Reference Filter based on the ECK entity ID.
4. Observe that ECK entity IDs are not available or functional in the filter options.

Proposed resolution

• Enhance the Views Reference Filter module to support filtering by entity IDs of ECK-based entities.
• Ensure proper detection of ECK entity types within the Views integration.
• Adjust filter logic to correctly handle references to ECK entity IDs.
• Optionally, expose ECK entity types as selectable options in the Views UI for the reference filter configuration.

A patch implementing this functionality has already been developed and is attached to this issue.

Remaining tasks

• Review and test the attached patch for compatibility and expected behavior.
• Provide feedback or suggestions for improvements.
• Commit the patch if approved.

User interface changes

• If applicable, ECK entity types will appear as options when configuring Views Reference Filters.

API changes

• No API changes are expected unless further integration with ECK's internal APIs becomes necessary.

Data model changes

• None.