polkadot-parachain simplifications and deduplications (#4916)

`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.

cumulus/polkadot-parachain/src/cli.rs

@@ -14,6 +14,7 @@
 // You should have received a copy of the GNU General Public License
 // along with Cumulus.  If not, see <http://www.gnu.org/licenses/>.
 
+use crate::common::NodeExtraArgs;
 use clap::{Command, CommandFactory, FromArgMatches};
 use sc_cli::SubstrateCli;
 use std::path::PathBuf;
@@ -94,6 +95,12 @@ pub struct Cli {
 	pub relay_chain_args: Vec<String>,
 }
 
+impl Cli {
+	pub(crate) fn node_extra_args(&self) -> NodeExtraArgs {
+		NodeExtraArgs { use_slot_based_consensus: self.experimental_use_slot_based }
+	}
+}
+
 #[derive(Debug)]
 pub struct RelayChainCli {
 	/// The actual relay chain cli object.