adding support for DISTINCT ON for postgres DB

[Chengxuan]

This PR adds support for PostgreSQL's DISTINCT ON clause to the firefly-common library, enabling efficient querying of distinct rows based on specific field combinations. This feature is PostgreSQL-specific and provides a more performant alternative to using GROUP BY or subqueries for deduplication scenarios.

Postgres ONLY

The feature explicitly validates that the database provider is PostgreSQL before applying DISTINCT ON, returning a clear error message for unsupported providers.

Usage Example

filter := queryFactory.NewFilter(ctx).
    DistinctOn("namespace", "type").
    Sort("created").
    And()

results, _, err := collection.GetMany(ctx, filter)

This generates SQL like:

SELECT DISTINCT ON (namespace, type) * 
FROM table 
ORDER BY namespace, type, created