Compose tcp.endpoint with network.Endpoint

### Description

transport/internal/network.Endpoint implements common facilities shared to send/receive datagrams over a network protocol. UDP, ICMP, RAW endpoints are composed with that type.

We can treat each TCP segment as a network datagram, so tcp.endpoint can be composed by a transport/internal/network.Endpoint. This will lead to reduced code duplication.

### Steps to reproduce

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