`polkadot-parachain` simplifications and deduplications

Details in the commit messages. Just copy-pasting the last commit
description since it introduces the biggest changes:

```
    Implement a more structured way to define a node spec
    
    - use traits instead of bounds for `rpc_ext_builder()`,
      `build_import_queue()`, `start_consensus()`
    - add a `NodeSpec` trait for defining the specifications of a node
    - deduplicate the code related to building a node's components /
      starting a node
```

The other changes are much smaller, most of them trivial and are
isolated in separate commits.