Decouple --defaults from non-interactive mode

[sisp]

While writing up my thoughts in #2680, I started wondering whether --defaults and running in non-interactive mode should be decoupled.

Currently, when we pass --defaults, Copier fails when there is any required question without a template's default answer, user's default answer, recorded answer, or programmatically provided answer via --data/--data-file. This means, we cannot use default answers for optional questions (i.e., no prompting) while being prompted for required questions. But I think that's a quite reasonable use case.

This affects copier copy for obvious reasons and copier update/copier recopy when a new template version introduces new required questions and new optional questions where we want to go with default answers for the latter (and for previously answered questions unless we pass the not-yet-available --ask flag).

I suggest to add a new flag --no-interaction or --non-interactive and change --defaults to not imply non-interactive mode anymore.

⚠️ This would be a breaking change!

/cc @lhupfeldt @bentheiii because of our somewhat related discussion in #2680

[lhupfeldt]

I agree that they should be decoupled.
I have been bitten by having used --defaults and --data... in our CI and then forgetting to add a new option with --data. Thus getting a default value which was wrong for the setup.
I think --non-interactive may be the more common flag. There is also --no-prompt.

[bentheiii]

I didn't realize --defaults implies non-interactive, seems very unintuitive, I'm fully on board with decoupling the two options.
I do however question why this would be a breaking change, AFAICT this would just make a case that resulted in an error before prompt a question now, I don't think that's a breaking change