Rewrite the light client JSON-RPC server

[tomaka]

cc #1415

This PR rewrites the light client JSON-RPC server in order to get rid of the json_rpc::services module of the library. This module was badly designed and is to be replaced with #1436.

Once #1436 is finished and merged, the code in this PR will be split into multiple different JSON-RPC servers (the idea right now being: one for the legacy API, one for the new API, one for transactions) that are merged together through the ServersMultiplexer.

The new code in this PR removes the security properties of the JSON-RPC server. They were never actually documented anywhere, and smoldot doesn't need to have a secure JSON-RPC server at the moment. These security properties will be restored after #1436.

Work time: 45h

[tomaka]

Note that I've been trying to keep the existing behavior as much as possible.

However, I remember instinctively changing some return values w.r.t. the new JSON-RPC API to better match the spec, but maybe not all of them. Unfortunately, I don't remember exactly what I've changed, so I don't know what to put in the CHANGELOG.
After this PR, I will do a review of the return codes so that they match the spec.

[tomaka]

While it's not unlikely that some bugs are being introduced, I'm reasonably confident in the code of this module, and thus I'm going to merge this once CI passes.