Only use `register_async_method` when needed

**Issue summary**

Most of the methods that we are registering in our RPC module are `async`. Some are not (e.g., `chain_get_message`) and can use `register_method` instead.

**Task summary**

- [ ] Make a list of those async functions
- [ ] Change them to synchronous functions and register them accordingly

**Other information and links**

https://docs.rs/jsonrpsee-core/0.21.0/jsonrpsee_core/server/struct.RpcModule.html#method.register_method