Add remote backend mode from bifrost-gateway

[lidel]

Filling here so we don't forget, upstream details in ipfs/boxo#576.

Once we have block / car backends in boxo/gateway, it will be easy to add config flag to rainbow which disables libp2p stack and only uses delegated retrieval via HTTP (trustless gateway + delegated routing).

I think the basic configuration options would be:

• RAINBOW_REMOTE_BACKEND with list of URLs of trustless gateways
• RAINBOW_REMOTE_BACKEND_MODE=block|car where block is implicit default (if not set)