SubConfig: allow to actually validate the nested configs

[smarie]

Fixes #2709

This is a fix for previous PR (reverted) #2710

SubConfig now has an optional constructor argument named ignore_validation. By default it is True to preserve retrocompatibility with legacy. Setting it to False will now correctly collect and raise validation errors and warnings for subconfig items.

[smarie]

Note that another way to fix this would be to make subconfig strict by default, but toggle the "non-strict" flag in the default config schema for the "extra" option. As you prefer, just let me know

[oprypin]

Thank you very much for the contribution. Finally I come back to ConfigOptions. And this makes perfect sense, particularly together with #2916 and #2938. I merged master (was needed after #2916) and added some of my own changes.

[smarie]

Thanks @oprypin !