fix: Leader as transaction ordering service

[SamHSmith]

Description

4914

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

[mversic]

you should follow the style of the code and add a new state to the block state machine. Let's consider this in more detail:

1. remove categorize_transactions function
2. make BlockBuilder::new -> BlockBuilder::chain -> BlockBuilder::sign produce SignedBlock(not ValidBlock)