adding nominee and nominator to interest types

[kathryn-ods]

Overview

• What does this pull request do? adds nominee and nominator codes to interest type
• How can a reviewer test or examine your changes?
• Who is best placed to review it?

Closes #

Translations

• [ ] New or edited strings appearing as a result of this PR will be picked up for translation
• [ ] I've notified the translation coordinator of any new strings that will need translating. See: https://openownership.github.io/bods-dev-handbook/translations.html

Documentation & Release

• [ ] I've checked that the documentation builds without failing. See: https://github.com/openownership/data-standard#building-the-documentation-locally
• [ ] I've thought about how and when this needs to be released. See: https://openownership.github.io/bods-dev-handbook/standard_releases.html
• [ ] I've updated the changelog, if necessary.
• [ ] I've updated reference.rst to reflect any changes to schema structure or naming.
• [ ] I've added or edited any other related documentation. See: https://github.com/openownership/bods-dev-handbook/blob/master/style_guide.md

[kathryn-ods]

Also: how pleasing are those 398 green dots when we run pytest :-))

I feel compelled to add 2 more tests to get a round number