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/book/usage/storage.rs

@@ -123,7 +123,7 @@
 //! ### Advanced
 //!
 //! For more advanced use cases, have a look at [`crate::storage::Storage::fetch_raw`] and
-//! [`crate::storage::Storage::fetch_keys`]. Both of these take raw bytes as arguments, which can be
+//! [`crate::storage::Storage::fetch_raw_keys`]. Both of these take raw bytes as arguments, which can be
 //! obtained from a [`crate::storage::StorageAddress`] by using
 //! [`crate::storage::StorageClient::address_bytes()`] or
 //! [`crate::storage::StorageClient::address_root_bytes()`].