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

Cargo.toml

@@ -36,6 +36,7 @@ documentation = "https://docs.rs/subxt"
 homepage = "https://www.parity.io/"
 
 [workspace.dependencies]
+async-trait = "0.1.72"
 assert_matches = "1.5.0"
 base58 = { version = "0.2.0" }
 bitvec = { version = "1", default-features = false }
@@ -100,7 +101,6 @@ sp-core = { version = "21.0.0", default-features = false }
 sp-core-hashing = "9.0.0"
 sp-runtime = "24.0.0"
 sp-keyring = "24.0.0"
-sp-version = "22.0.0"
 
 # Subxt workspace crates:
 subxt = { version = "0.31.0", path = "subxt", default-features = false }