* validation support for unions and interfaces
* refactor SQL generation logic for improved readability
* '/v1/relay' endpoint for relay schema
* implement 'Node' interface and top level 'node' field resolver
* add relay toggle on graphiql
* fix explain api response & index plan id with query type
* add hasura mutations to relay
* add relay pytests
* update CHANGELOG.md
Co-authored-by: rakeshkky <12475069+rakeshkky@users.noreply.github.com>
Co-authored-by: Rishichandra Wawhal <rishi@hasura.io>
Co-authored-by: Rikin Kachhia <54616969+rikinsk@users.noreply.github.com>
