Allow integrated relay chain light client

[skunert]

In this PR, I added a command line flag --relay-chain-rpc-client to enable experimental support for an embedded light client.

This leaves us with three execution modes for the relay chain:

1. Embedded polkadot full node
2. External polkadot node (using embedded mini-node for collation and pov-recovery)
3. Embedded light-client (using embedded mini-node for collation and pov-recovery)

Implementation Overview

• I reused the existing RPC infrastructure as much as possible. Only the worker is switched out
• The light client is run in-process and gets the chain spec that is passed via relay chain args to the collator
• We communicate with the help of the jsonrpsee library and let it handle subscription and request management
• The example light-client implementation in the smoldot repo uses an implementation of the platform trait based on async-std. Since we use tokio here, I converted the implementation 1:1 using tokio structures.

Remaining issues

This issue is impacting us, I will take a look at what needs to be done on the substrate side:

• Light-node stops reporting finalized headers for certain chain-specs smol-dot/smoldot#239

[dmitry-markin]

LGTM as far as I understood the underlying logic from the code of PR.

[davxy]

LGTM mostly nitpicks