LightClient: Higher level API for multi-chain usecase

[lexnv]

We currently have:

• high level API for targeting just one chain
• very low level API for gaining control to the smoldot client

We would like to introduce something in between, that would allow users to easily build a multichain smoldot client without having to understand the low-level API.

For more context: #1238 (comment)

let client: MediumLightClient = MediumLightClient::builder()
.add_chain(
AddChianBuilder::with_identifier( "polkadot" )
.with_spec( include_str!( "../assets/polkadot_chain_spec.json")
)
.add_chain(
AddChainBuilder::with_identifier( "asset-hub")
.relay_chain( "polkadot")
.from_url("ws://127.0.0.1:9944")
)
.build().await;
let polkadot_api = client.for_chain( "polkadot");

[jsdw]

Closed by #1475 I think :)