Allow to set debugger path to use

[peace-maker]

gdb or gdb-multiarch is hardcoded in the gdb module. Right now you'd have to create a pwntools-gdb symlink in the path which takes precedence. Allow to override the default in the exploit script by adding some property to the context.

This can be useful when having multiple gdb versions installed or standalone installs of e.g. pwndbg coming with a pwndbg executable instead of gdb.

Specifying the path to the debugger might even be necessary on Windows, where the debugger binary is often not in the PATH.