Added new checks to add_assignment (#2332)

We now check for every requirement of `add_assignment` described in [RFC
5](https://github.com/polkadot-fellows/RFCs/blob/gav-corejam/text/0005-coretime-interface.md).
These are:

- Not more than 100 assignments in a schedule
- Assignment parts add up to 57600 (neither overscheduled nor
underscheduled)
- Starting blocks must be greater than or equal to current block + 10
when schedules are added
- Assignments within an added set must be unique
- Assignments within an added set must be sorted