Implement generating a chain spec

[tomaka]

[tomaka]

The motivation behind this issue was to support generating chain checkpoints using smoldot.

Unfortunately, the format of checkpoints isn't clearly defined and is really weird. Substrate at the moment generates a bunch of extra data that smoldot doesn't even try to interpret. Since Substrate is the only implementation that can generate checkpoints at the moment, it is obvious that any checkpoint generate by Substrate is accepted by all implementations. However, because the format isn't clearly defined, generating a checkpoint from smoldot might run the risk that other implementations are unable to parse it.

I'm going to tag this as "blocked", waiting for a proper checkpoint format to be defined: https://github.com/paritytech/substrate/issues/11184