feat!: Require generation in the multi-keyring

[robin-aws]

Fixes #114

Breaking change since multi-keyrings currently allow a generator to be configured and yet accept encryption materials that already have a plaintext data key defined.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[acioc]

Looks good! There's no "Reference-level Explanation" (not sure if there needs to be), but that's fine since the reference-level is your spec change anyways.

[robin-aws]

Looks good! There's no "Reference-level Explanation" (not sure if there needs to be), but that's fine since the reference-level is your spec change anyways.

Yes, in this case there's no detail that would be in one but not the other. Would it be better to label the section as "Guide-level/Reference-level Explanation"?

[acioc]

Looks good! There's no "Reference-level Explanation" (not sure if there needs to be), but that's fine since the reference-level is your spec change anyways.

Yes, in this case there's no detail that would be in one but not the other. Would it be better to label the section as "Guide-level/Reference-level Explanation"?

I personally prefer renaming - I think renaming makes it clear it wasn't forgotten about and reinforces the standard we're trying to follow.

[acioc]

I personally preferred the wording changes on Security Implications but it's not a blocker.