Remove rococo runtime dependencies

[sulijia]

In order to obtain events on the rococo relay chain, the order pallet introduces the rococo runtime dependency.
In order to submit signed transactions to the rococo relay chain, the node code introduces the rococo runtime dependency.
We should remove this dependency, otherwise upgrading rococo or switching to connecting to polkadot will be affected.
The work that needs to be completed is:

1. The OnDemandOrderPlaced event on the relay chain can be parsed in the order pallet. The source code that needs to be replaced is as follows:
   let v_price: Vec = head_data
   .iter()
   .filter_map(|item| {
   if let rococo_runtime::RuntimeEvent::OnDemandAssignmentProvider(
   parachains_assigner_on_demand::Event::OnDemandOrderPlaced {
   para_id: pid,
   spot_price: sprice,
   },
   ) = item.event
   {
   if pid == para_id {
   Some(sprice)
   } else {
   None
   }
   } else {
   None
   }
   })
   .collect();
2. Able to complete offline signature in node without relying on rococo runtime, for example, you can use subxt

[sulijia]

Import runtime Call / Storage metadata without import the whole runtime
https://github.com/paritytech/substrate/issues/8158