feat: add validation to merged local/inline configs

[Roshan-R]

Closes #275

[travier]

Mostly LGTM. A couple of small changes.

[travier]

Can you also copy this validation code to all stable specs? See #476 (comment):

This validation isn't FCOS-specific; it applies to all variants. So, it should be in base. And since we're only going to be producing errors for configs that were already invalid (but would be rejected when the machine boots rather than when Butane runs) we should backport it to stable base specs.

[Roshan-R]

Can you also copy this validation code to all stable specs? See #476 (comment):

This validation isn't FCOS-specific; it applies to all variants. So, it should be in base. And since we're only going to be producing errors for configs that were already invalid (but would be rejected when the machine boots rather than when Butane runs) we should backport it to stable base specs.

I have done the backporting to all the versions except v1 since It does not have this feature

[prestist]

Thank you for working on this; these changes make sense to me, and LGTM, one small nit, not needed but would be nice. The other nit is can we maybe cleanup the commit history a bit, i.e squash redundancies.

[Roshan-R]

Commits have been squashed and all requested changes have been addressed.
Let me know if there's anything else needed

[prestist]

Thank you for doing this; it lgtm well done!