listener refactoring: maximize reuse without forcing between the different modes (#75)

[makdeniss]

This change simplifies the Gateway and Listener codebase by centralizing dependency injection for IO and schema resolvers, and unifying client creation logic, thereby maximizing code reuse across Standard and KCP operational modes.

Changes

• Consolidated client creation in TargetCluster, removing explicit KCP branching from the main connection logic.
• Moved IOHandler and SchemaResolver initialization to cmd/listener.go, reducing duplication.
• Updated ReconcilerOpts to inject shared dependencies, enabling consistent behavior across both Standard and KCP modes.