[Feature Request]: use app's gRPC for everything and drop requiring core RPC

### Implementation ideas

- list which end-points are currently not supported by app's grpc
- implement those in app (e.g. see: https://github.com/celestiaorg/celestia-app/issues/3421)
- use these in node
- profit: remove confusing CLI flags, remove docs around core RPC from node

related: https://github.com/celestiaorg/celestia-node/issues/3357