[Feature Request] Advanced Campaign Segmentation using Custom SQL/JSON Queries with Dry-Run Preview

Description Problem / Context Currently, Listmonk lacks a native campaign segmentation mechanism within the campaign creation workflow. When organizing subscribers based on metadata—such as language (lang), tenant/organization (tenant_id), custom user attributes, or behavioral properties—the current platform limits users to targeting fixed lists. This restriction introduces significant operational challenges: • Inability to Perform User/Language/Tenant-Level Targeting: Senders cannot dynamically filter a broad list down to specific subsets (e.g., users where attribs.lang = 'tr' and attribs.tenant = 'tenant_a') at the campaign level. • List Proliferation & Overhead: Managing static lists for every combination of language, region, or tenant leads to list bloat and high synchronization maintenance. • Lack of Audience Visibility (No Dry Run): Senders have no immediate feedback or dry-run validation to verify how many subscribers match their crit…