Add block_no to Checkpoint

[paluh]

Some context:

• This is a draft opened to support related discussion: Add block height to the matches #213
• This PR was manually coded (no AI).
• Tests were executed against cardano-node using SQLite. PostgreSQL and Ogmios will be next if the overall change is approved.

Main changes:

• I introduced Checkpoint which is a ~Checkpoint Point BlockNo. I introduced that type on both Database and App levels.
• I added block_no to the checkpoints table.
• Checkpoint is used during insertions and during checkpoint and match related retrievals.
• Point is used in rollback handling and in queries.