tcp: (fixes #1309) add support for configurable TCP socket base

This MR updates TcpL4Protocol to allow the TCP socket base class to be configurable. The TCP socket is instantiated using ObjectFactory, with TcpSocketBase remaining the default.

This change enables the use of TCP socket implementations that inherit from TcpSocketBase and can be selected explicitly, while preserving existing behavior when no custom socket type is configured.

The change is motivated by an rLEDBAT implementation, where a custom socket overrides ReceivedData() to access TCP header information and AdvertisedWindowSize() to control the advertised receive window.