feat(design): create DaffOpenableDirective

[xelaint]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/graycoreio/daffodil/blob/develop/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Part of: #763
Fixes: #3020

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[xelaint]

@damienwebdev Need to finalize what the output naming is for the openable directive. Are we sticking to toggled?

[xelaint]

@griest024 we made some updates according to your concerns. lmk what you think.

[xelaint]

@griest024 had to fix some tests