agent, server: improve packet framing and use TLS 1.3

This pull request refactors the TLS framing and buffer management in the `Link` class to improve correctness and maintainability, and updates the SSL context initialization to use TLS 1.3 for enhanced security. CloudStack uses a 4-byte header for TLS packets. Earlier, it was not sent within the TLS application data, which affected maintainability and the implementation of agent-server communication using a different language. The most important changes are grouped below.

* Reworked the TLS buffer handling in `Link.java`, replacing legacy header and packet assembly logic with a more robust system using `netBuffer`, `appBuffer`, and an explicit `headerBuffer` for frame length management. This improves frame parsing and avoids buffer overflows.
* Refactored the read and write logic: the `read` method now correctly assembles frames from TLS streams, handling buffer resizing and edge cases, while the `doWrite` method builds TLS packets with a 4-byte length header and payload, ensuring correct framing and handshake handling.
* Simplified the message sending and writing logic by removing manual header prepending and using the new framing system; the write queue now contains only payload buffers, and the header is added during the TLS wrap process.

* Updated SSL context initialization in `Link.java` to use `SSLUtils.getSSLContextWithLatestVersion()`, ensuring that TLS 1.3 is used for all server, client, and management SSL contexts.
* Added a new method `getSSLContextWithLatestVersion()` in `SSLUtils.java`, which returns an `SSLContext` instance for TLS 1.3.

Signed-off-by: Abhishek Kumar <[email protected]>

Author: shwstppr