`polkadot-parachain`: add manual seal support (#5586)

Resolves https://github.com/paritytech/polkadot-sdk/issues/5026

This PR adds support for starting a dev node with manual seal consensus.
This can be done by using the `--dev-block-time` argument . For example:
```
polkadot-parachain --chain asset-hub-rococo-dev --dev-block-time 5000 --tmp
```