`polkadot-parachain-bin`: small cosmetics and improvements (#4666)

Related to: https://github.com/paritytech/polkadot-sdk/issues/5

A couple of cosmetics and improvements related to
`polkadot-parachain-bin`:

- Adding some convenience traits in order to avoid declaring long
duplicate bounds
- Specifically check if the runtime exposes `AuraApi` when executing
`start_lookahead_aura_consensus()`
- Some fixes for the `RelayChainCli`. Details in the commits description