FR: Conway vote validity field

**Describe the feature you'd like**
`voting_procedure` table currently capture all votes cast for governance proposals. However, almost always what you want to query is valid votes cast for a specific proposal. With a vote being invalidated by for example a later vote cast from the same entity on the same proposal or if entity de-register before the propsal is finalized. There might be other factors that control a votes validity like possibly the DReps activity state. I'm not sure if its possible or not, but I would like to propose to include an `is_valid` field in `voting_procedure` table that shows if the vote is/was included in "propsal snapshot".

**Describe alternatives you've considered**
Each consumer(API layers) of DBSync has to create complex logic and joins to figure out what votes are valid and considered for a proposal.