Support specification of strict on Enum type fields

[sydney-runkle]

Change Summary

Support specification of strict on Enum type fields
More broadly, this allows for the strict specification to be added to lax-or-strict schema, where it's expected but for some reason wasn't allowed to be added.

Fix #7759

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @lig

[sydney-runkle]

Please review

[davidhewitt]

Fix looks good to me.

I half wonder whether there's a way that we could inspect the core_schema methods to figure out what the allowed constraints could be (but probably that's a complex algorithm which isn't great to run on startup time).