Related to https://github.com/paritytech/polkadot-sdk/issues/4787

The main changes in this PR are the following:
- making the NodeSpec logic generic on the Block type
- adding an omni-node variant with u64 block number

Apart from this, the PR also moves some of the logic in `service.rs` to
the `common` subfolder

The omni-node variant with u64 block number is not used yet. We have to
either expose the option in the CLI or to read the block number from the
chain spec somehow. Will do it in a future PR.