cli: Command to fetch chainSpec and optimise its size

[lexnv]

Add a cli command that:

• fetches the chainSpec needed to start a light client from a running node
• add an extra config to enable / disable chainSpec size optimization

The size optimization can be done by hashing the genesis.raw entries, obtaining as a result a stateRootHash that will drastically reduce the chainSpec size; and allow for faster light client initialization.

Then, this CLI tool together with scripts/artifacts.sh can run in a cronjob to automatically update:

• demo chain specs
• metadata files
• codegen polkadot.rs file