Define mandatory field with checker rules

[ClemensLinnhoff]

#### Reference to a related issue in the repository

750

Add a description

• Add is_set to rules.yml
• Add is_set as rule to OSI fields that shall be mandatory to set
• Remove "All fields have to be set" from OSI documentation

Take this checklist as orientation for yourself, if this PR is ready for the Change Control Board:

• [x] My suggestion follows the style and contributors guidelines.
• [x] I have taken care about the message documentation and the fields and enums documentation.
• [x] I have done the DCO signoff.
• [x] My changes generate no errors when passing CI tests.
• [x] I have successfully implemented and tested my fix/feature locally.
• [x] Appropriate reviewer(s) are assigned.

If you can’t check all of them, please explain why. If all boxes are checked or commented and you have achieved at least one positive review, you can assign the label ReadyForCCBReview!

[pmai]

CCB 2024-02-26: Merge as-is, thanks for the effort.