M704 run custom g-code macro

[dragonnn]

add M704 command to run custom macro from current loaded filament
This is a follow to my post https://forum.duet3d.com/topic/8887/run-custom-macro-for-loaded-filament
I deiced I will give a try and implement it. Seems to work fine, even when the file isn't found or no filament is loaded.

[dc42]

Thank you for your contribution. Your PW appears to define a new code M704 which works mostly like M703 except that a custom filament can be chosen instead of config.g. Instead of introducing a new GCode, please can you look at the whether the following alternatives would meet your requirement:

1. Using conditional GCode to execute a M98 command, where the filename is constructed from the directory of the current filament and the required filename;
2. Add an optional string parameter for the filename to the M703 command, with the default being config.g. This might not be exactly what you want because the current M703 implementation checks that the filament is loaded too.