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

examples/wasm-example/src/routes/signing.rs

@@ -162,9 +162,9 @@ impl Component for SigningExamplesComponent {
                                 // Apply the signature
                                 let signed_extrinsic = partial_signed.sign_with_address_and_signature(&account_id.into(), &multi_signature);
 
-                                // do a dry run (to debug in the js console if the extrinsic would work)
-                                let dry_res = signed_extrinsic.dry_run(None).await;
-                                web_sys::console::log_1(&format!("Dry Run Result: {:?}", dry_res).into());
+                                // check the TX validity (to debug in the js console if the extrinsic would work)
+                                let dry_res = signed_extrinsic.validate().await;
+                                web_sys::console::log_1(&format!("Validation Result: {:?}", dry_res).into());
 
                                 // return the signature and signed extrinsic
                                 Message::ReceivedSignature(multi_signature, signed_extrinsic)