Support for jlink gdb server

Valinor currently support only pyOCD gdb server at the moment, which works for platforms with cmsis-dap debuggers.
We could add jlink gdb (or later any other in the future). The decision which one to run automatically, will be made based on fetching progen target definition to get default debugger.

Start by locating the existing pyOCD GDB-server support and the code that fetches the ProGen target definition. Determine how the default debugger is represented and where server selection occurs. Done means jlink GDB-server support works and selection chooses it from the target definition without breaking pyOCD support.

Written by the indexing model from the issue text.