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/client/offline_client.rs

@@ -4,9 +4,9 @@
 
 use crate::custom_values::CustomValuesClient;
 use crate::{
-    blocks::BlocksClient, constants::ConstantsClient, events::EventsClient,
-    rpc::types::RuntimeVersion, runtime_api::RuntimeApiClient, storage::StorageClient,
-    tx::TxClient, Config, Metadata,
+    backend::RuntimeVersion, blocks::BlocksClient, constants::ConstantsClient,
+    events::EventsClient, runtime_api::RuntimeApiClient, storage::StorageClient, tx::TxClient,
+    Config, Metadata,
 };
 use derivative::Derivative;