Feature: BackedEnumFilter allow multiple values similar to SearchFilter[exact]

[MeronNagy]

Description
Currently, the BackedEnumFilter in API Platform only supports filtering by a single enum value. This differs from the behavior of SearchFilter[exact], which can handle multiple values.

When migrating from string constants to backed enums, it would be nice if BackedEnumFilter could be a plug in solution for SearchFilter[exact].

Example

Before:

When using BackedEnumFilter, you can only filter by a single enum value:

GET /books?status=published

After:

Support both single values and multiple values

GET /books?status=published
GET /books?status[]=published&status[]=in_review

The implementation:

1. Modifies filterProperty to accept array values and use IN SQL operator
2. Updates getDescription to document both single value and array notation (propertyName[])
3. Maintains backward compatibility with existing usage

I will link a PR shortly.