Support Dijkstra Era

Presently, some of our functions are hard-coded to ConwayEra; e.g. https://github.com/cardano-scaling/hydra/blob/master/hydra-node/src/Hydra/Chain/CardanoClient.hs#L233, in order to support Dijkastra we need to remove this kind of assumption.

### Potential solutions?

- Perhaps one idea is to pass around the era a bit more clearly? (Era witnesses)
- ? Stop using hydra-cardano-api