Add FreeRTOS echo-server example

[LinuxJedi]

Runs in Linux, should run in Harmony too, but not tested there.

[Copilot]

Pull request overview

Adds a new echo-server example demonstrating a minimal wolfSSH echo server running on FreeRTOS (POSIX/Linux simulator) with FreeRTOS-Plus-TCP over a libpcap network interface, and wires in the needed build/docs scaffolding.

Changes:

• Adds a new echo-server/ example (FreeRTOS configs, wolfSSH echo server logic, and FreeRTOS-Plus-TCP IO callbacks).
• Adds a Makefile-based build for Linux/POSIX and documentation for veth/libpcap network setup and usage.
• Adds new git submodules for wolfSSL, wolfSSH, FreeRTOS-Kernel, and FreeRTOS-Plus-TCP under echo-server/, and links the example from the top-level README.

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
README.md	Adds the new example entry and description.
.gitmodules	Adds submodules required to build the new example.
echo-server/README.md	Build/run instructions and Linux veth/libpcap network setup guide.
echo-server/Makefile	Make-based build that compiles wolfSSL/wolfSSH + FreeRTOS kernel + FreeRTOS-Plus-TCP into static libs and links the example.
echo-server/main.c	FreeRTOS entry point, network bring-up, and single-client accept loop.
echo-server/echo_server.c / echo_server.h	wolfSSH server initialization, authentication, accept, and echo loop.
echo-server/freertos_tcp_io.c / freertos_tcp_io.h	wolfSSH IO callbacks implemented via FreeRTOS-Plus-TCP sockets.
echo-server/FreeRTOSConfig.h	FreeRTOS kernel configuration for the POSIX port.
echo-server/FreeRTOSIPConfig.h	FreeRTOS-Plus-TCP configuration for the Linux libpcap interface.
echo-server/user_settings.h	wolfSSL/wolfSSH build configuration for this example.
echo-server/.gitignore	Ignores build artifacts for the example.

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[LinuxJedi]

Fixed Copilot reports 1 & 2, the rest aren't important for an example.