scripts: west_commands: runners: add ST-LINK GDB server

Add the ST-Link GDB server (part of the STM32CubeCLT) as west runner.

The STM32CubeCLT is an all-in-one multi-OS command-line toolset, which is
part of the STM32Cube ecosystem, and notably includes a GDB server for
debugging using ST-Link probes.

This runner supports the "attach", "debug" and "debugserver" commands.

Documentation: https://www.st.com/en/development-tools/stm32cubeclt.html


Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>