Make templates node building optional

[iulianbarbu]

The node shouldn't compile by default when you do cargo build. It should be feature gated behind --features with-node. By default, the template should only compile the runtime and produce a wasm. Instead, the user is invited to download polkadot-omni-node/polkadot-parachain.

To run this wasm, one is expected to use either of the options below, so test them too:

• Omni-node: add support for starting a dev chain with manual seal #5026
• polkadot-omni-node + zombienet
• chopsticks.yaml

Do it for:

• minimal
• parachain