Rewrite sov_ledger_rpc::client::RpcClient without jsonrpsee macros

[neysofu]

The current ledger JSON-RPC client is auto-generated with jsonrpsee's rpc macro. This is very handy, but has two drawbacks:

1. Some parameter serialization patterns are not supported by jsonrpsee, so we're forced to resort to hacks on the server side. See feat(rpc): implement ledger_getEvents #1058 for some context.
2. Somewhat related to n.1, serde details are exposed in the public API even though we'd like not to e.g. HexHash. A custom trait would solve this problem by accepting the right type of arguments ([u8; 32]), and the inner serialization logic can then become an implementation detail.

Once this is done, we also need to:

• Remove the ledger_getEvents hack on the server-side.
• Remove the test for said hack.