Allow changing meson options for packages

[nazar-pc]

#1353 is an example of a change that is only possible with gvsbuild modification.

It would be nice to have an ability to change those options with CLI as well, for example --meson-option gtk4::build-testsuite=false or something along those lines. That way it would be possible to enable optional features and disable those enabled by default without modifying gvsbuilt itself.