[secure-transport] introduce SecureSession class (openthread#11046)

This commit introduces the `SecureSession` class to manage
session-related functionality, decoupling it from the
`SecureTransport` class. `SecureSession` provides method such as
`Connect()`, `Send()`, and `IsConnected()`, while `SecureTransport`
focuses on transport-level operations and common configuration
(e.g., setting PSK, cipher keys).

The `Dtls` and `Tls` subclasses now inherit from both
`SecureTransport` and `SecureSession`, effectively providing the same
methods as before and implementing a single DTLS/TLS session over a
socket.