Refactor RPC part to make it easier to add RPC modules for downstream implementors

[clangenb]

Example of how to create an aggregated rpc module: https://github.com/paritytech/substrate/blob/eddf888394cba28b048bd79e319e8b27f4b414f3/bin/node-template/node/src/rpc.rs#L49.

Example of how to use the proc_macro to generate client and server: https://github.com/paritytech/jsonrpsee/blob/master/proc-macros/src/lib.rs#L331.

Working Example for encointer-node:

1. Create Rpc-client/-server: https://github.com/encointer/pallets/blob/master/communities/rpc/src/lib.rs
2. Add rpc server module at a well-defined place: https://github.com/encointer/encointer-node/blob/9e603970b0aa828497408dcd8554ce2ee9f25cee/node/src/rpc.rs#L86

So instead of the bloat we have here I want to see:

pub fn public_api_rpc_handler<R, G>(top_pool_author: Arc<R>, getter_executor: Arc<G>) -> RpcModule<()> {

    let mut module = RpcModule::new(());
    module.merge(SomeEnclaveRpc::new(some_params).into_rpc())?;

   ...
}

Caveat:
jsonrpsee is not no_std, so we either need to maintain an sgx fork, or we wait until we have migrated to teaclave-v2.0.0. The latter is preferred, as we want to do it soon anyhow: #1102