Implement Configurable TCP Keepalive Settings in PJSIP Transports

This commit introduces configurable TCP keepalive settings for both TCP and TLS transports. The changes allow for finer control over TCP connection keepalives, enhancing stability and reliability in environments prone to connection timeouts or where intermediate devices may prematurely close idle connections. This has proven necessary and has already been tested in production in several specialized environments where access to the underlying transport is unreliable in ways invisible to the operating system directly, so these keepalive and timeout mechanisms are necessary.

Fixes #657

(cherry picked from commit 3d40d3427106fc751d77d2d5f182a92becfdd53a)