Introduce Backend trait to allow different RPC (or other) backends to be implemented (#1126)

* WIP backend trait

* WIP converting higher level stuff to using Backend impl

* more implementing new backend trait, mainly storage focused

* Get core code compiling with new backend bits

* subxt crate checks passing

* fix tests

* cargo fmt

* clippy/fixes

* merging and other fixes

* fix test

* fix lightclient code

* Fix some broken doc links

* another book link fix

* fix broken test when moving default_rpc_client

* fix dry_run test

* fix more tests; lightclient and wasm

* fix wasm tests

* fix some doc examples

* use next() instead of next_item()

* missing next_item() -> next()s

* move legacy RPc methods to LegacyRpcMethods type to host generic param instead of RpcClient

* standardise on all RpcClient types prefixed with Rpc, and 'raw' trait types prefixed with RawRpc so it's less ocnfusing which is which

* rename fixes

* doc fixes

* Add back system_dryRun RPC method and rename tx.dry_run() to tx.validate(), to signal that the calls are different

* Add a test that we return the correct extrinsic hash from submit()

* add TransactionValid details back, and protect against out of range bytes

* add test for decoding transaction validation from empty bytes

* fix clippy warning

subxt/src/custom_values/custom_values_client.rs

@@ -47,15 +47,14 @@ impl<T: Config, Client: OfflineClientT<T>> CustomValuesClient<T, Client> {
 
 #[cfg(test)]
 mod tests {
-    use std::collections::BTreeMap;
-
+    use crate::backend::RuntimeVersion;
     use crate::custom_values::CustomValuesClient;
-    use crate::rpc::types::RuntimeVersion;
     use crate::{Metadata, OfflineClient, SubstrateConfig};
+    use codec::Encode;
     use scale_decode::DecodeAsType;
     use scale_info::form::PortableForm;
     use scale_info::TypeInfo;
-    use sp_core::Encode;
+    use std::collections::BTreeMap;
 
     #[derive(Debug, Clone, PartialEq, Eq, Encode, TypeInfo, DecodeAsType)]
     pub struct Person {
@@ -116,7 +115,6 @@ mod tests {
             RuntimeVersion {
                 spec_version: 0,
                 transaction_version: 0,
-                other: Default::default(),
             },
             mock_metadata(),
         );