feat: add light client support

[evilrobot-01]

Still very much a work in progress, but creating this draft PR in case anyone wants to try it out.

cargo run --features=light-client -- follow chain -r ./tests/polkadot.json -p ./tests/polkadot-asset-hub.json

The chainspecs can be sourced from https://github.com/smol-dot/smoldot/tree/main/demo-chain-specs.

Takes a few seconds to start, butt you should then start to see blocks flowing in:

Polkadot: #19,867,305, parent hash: 0x9c4d2b5672c3ae9df60afe3e25665d194a870224266fef535ea582448e027c07, state root: 0x1d241b1f70955d92d423e26954a190fb9979e5558b1662fc739ac07155912fcb, extrinsics root: 0xe6ce7891499d3d242ac009e28085831e1c54c6db6d7398318bf7a560fdb32005
Polkadot: #19,867,306, parent hash: 0xf13b5662d092412d8053b60047f916e809f6e9e2535a32bd149ac4fb3c53a88b, state root: 0x2d354a91fe39b07e6fe7924f85bcde6a442ce9790c695fb3920dec376e5cd1fa, extrinsics root: 0x67c96e35313f277bc0640422804853182a8af3b015ae5ed419a282dda9012142
Polkadot Asset Hub: #5,845,019, parent hash: 0x9f2a5dc303ba92117f8d7f3daf851f9374351795212dba2f3fd6c1f05ae86296, state root: 0x7b1fcad27dca9b789efdaf9b06e415b7573d66c02dc99542d121b58df6f93e76, extrinsics root: 0x9a0d0736484058bed00785ac8e22392a2aa76a7e9afb2dcd058995e41127f9e9
Polkadot Asset Hub: #5,845,020, parent hash: 0x025804268b1c92d25118c21ead06dfa0976bb8288f19475f8e59fe038c3bec42, state root: 0x896d9fb1eb572ba0ba015db183f2fa40827dd0b7385e67c4ef31c1e243811e1c, extrinsics root: 0x73726cdc48f859f9ed1a0119f9e3e72d381f03ddf81986acb86d34b22d917c83

Goals are to enable tracing, useful for debugging connectivity and potentially a nice feature to have running in a background terminal.

[Daanvdplas]

Curious on your thoughts whether we want to use https://github.com/paritytech/json-rpc-interface-spec ?

[evilrobot-01]

Ideally, but depends on whether there is any interest in this beyond me. It was put together pretty quickly thus far.

[AlexD10S]

To improve user' UX we should download the chainspecs. Maybe -r polkadot or -r rococo and get the polkadot.json directly.

Does pop connect instead of pop follow makes sense?

Is nice how easy to is to run a light client with pop, but hard for me to see real use cases, What kind of users do you think will use this feature?