fix: remove serde(flatten) from SetKeyValue/RemoveKeyValue

[mversic]

Description

serde cannot flatten Strings and O::Id is always serialized as string. As a consequence it's impossible to serialize/deserialize SetKeyValue/RemoveKeyValue in genesis

Linked issue

Closes #{issue_number}

Benefits

Checklist

• I've read CONTRIBUTING.md
• I've used the standard signed-off commit format (or will squash just before merging)
• All applicable CI checks pass (or I promised to make them pass later)
• (optional) I've written unit tests for the code changes
• I replied to all comments after code review, marking all implemented changes with thumbs up