Add weak custom CLI hook to extend CommonCLI without forking core commands

[piotrmalek]

Summary

This PR adds an optional, weakly-linked hook to CommonCLI::handleCommand():

• extern "C" bool meshcore_custom_cli(uint32_t sender_ts, const char* cmd, char* reply)
• If the hook returns true, the command is considered handled and the standard CLI processing is skipped.
• Default implementation returns false, so existing behavior is unchanged.

Motivation

Board / variant-specific builds sometimes need to add a few extra CLI commands without modifying upstream CLI code paths each time.
A weak symbol hook allows vendors and custom forks to ship additional commands as a separate translation unit, while keeping the upstream CLI intact.

Backward compatibility

• No existing commands are modified.
• If no custom implementation is provided, the weak default returns false and the standard CLI behaves exactly as before.

Example usage

Create e.g. customCli.cpp in your board/variant build and provide your own implementation:

#include <cstring>
#include <cstdio>
#include <cstdlib>

extern "C" bool meshcore_custom_cli(uint32_t sender_ts, const char* cmd, char* reply) {
  (void)sender_ts;

  if (strcmp(cmd, "pingx") == 0) {
    std::snprintf(reply, 64, "PONGx!");
    return true;
  }

  return false; // let it handle the standard CLI
}