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/lib.rs

@@ -63,6 +63,7 @@ use tracing_subscriber as _;
 #[allow(unused_imports)]
 pub use getrandom as _;
 
+pub mod backend;
 pub mod blocks;
 pub mod client;
 pub mod config;
@@ -72,7 +73,6 @@ pub mod dynamic;
 pub mod error;
 pub mod events;
 pub mod metadata;
-pub mod rpc;
 pub mod runtime_api;
 pub mod storage;
 pub mod tx;
@@ -91,6 +91,7 @@ pub use crate::{
 pub mod ext {
     pub use codec;
     pub use frame_metadata;
+    pub use futures;
     pub use scale_bits;
     pub use scale_decode;
     pub use scale_encode;