cmake: support custom network interfaces

[HTRamsey]

Description

Allow CMake consumers to define a project-specific freertos_plus_tcp_network_if target before adding FreeRTOS-Plus-TCP.

This change:

• validates that the custom target exists when A_CUSTOM_NETWORK_IF is selected;
• centralizes the dependencies shared by every network-interface target in the base NetworkInterface/CMakeLists.txt;
• keeps platform-specific dependencies in each individual interface;
• restores the STM32FXX and STM32HXX legacy CMake selections;
• adds intermediate CMake dispatchers for STM32 legacy and third-party interfaces;
• corrects the ATSAM4E selector and updates the CMake consumer documentation.

Applications providing a custom interface now only need to attach their own platform-specific sources and libraries. FreeRTOS-Plus-TCP supplies its internal target dependencies consistently.

Test Steps

• Configured and built the LOOPBACK network-interface target with the GCC POSIX FreeRTOS port.
• Configured both STM32FXX and STM32HXX legacy selections.
• Configured and built a standalone CMake consumer that defines freertos_plus_tcp_network_if before adding FreeRTOS-Plus-TCP.
• Ran git diff --check.

Checklist:

• I have tested my changes. No regression in existing tests.
• I have modified and/or added unit-tests to cover the code changes in this Pull Request.

Related Issue

None.

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