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.lock

@@ -1987,7 +1987,6 @@ dependencies = [
  "regex",
  "scale-info",
  "sp-core",
- "sp-runtime",
  "substrate-runner",
  "subxt",
  "subxt-codegen",
@@ -2581,12 +2580,6 @@ dependencies = [
  "syn 1.0.109",
 ]
 
-[[package]]
-name = "parity-wasm"
-version = "0.45.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e1ad0aff30c1da14b1254fcb2af73e1fa9a28670e584a626f53a369d0e157304"
-
 [[package]]
 name = "parking"
 version = "2.1.0"
@@ -3842,18 +3835,6 @@ dependencies = [
  "twox-hash",
 ]
 
-[[package]]
-name = "sp-core-hashing-proc-macro"
-version = "9.0.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "727d7c7d42e59a13eb1412758630569dd6c3640c4754d9a948e8d943f99bc932"
-dependencies = [
- "proc-macro2",
- "quote",
- "sp-core-hashing",
- "syn 2.0.28",
-]
-
 [[package]]
 name = "sp-debug-derive"
 version = "8.0.0"
@@ -4074,36 +4055,6 @@ dependencies = [
  "trie-root",
 ]
 
-[[package]]
-name = "sp-version"
-version = "22.0.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1a49e88372c9d923e95dc855f1e7bdbca6638574d59a8c3cb9f7e5ecea638481"
-dependencies = [
- "impl-serde",
- "parity-scale-codec",
- "parity-wasm",
- "scale-info",
- "serde",
- "sp-core-hashing-proc-macro",
- "sp-runtime",
- "sp-std",
- "sp-version-proc-macro",
- "thiserror",
-]
-
-[[package]]
-name = "sp-version-proc-macro"
-version = "8.0.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "39e1e11ab230fd0ede7aefeb246a1ef7e6af58634c490a2d911ce71aabc0718a"
-dependencies = [
- "parity-scale-codec",
- "proc-macro2",
- "quote",
- "syn 2.0.28",
-]
-
 [[package]]
 name = "sp-wasm-interface"
 version = "14.0.0"
@@ -4235,6 +4186,7 @@ name = "subxt"
 version = "0.31.0"
 dependencies = [
  "assert_matches",
+ "async-trait",
  "base58",
  "bitvec",
  "blake2",
@@ -4259,7 +4211,6 @@ dependencies = [
  "sp-core-hashing",
  "sp-keyring",
  "sp-runtime",
- "sp-version",
  "subxt-lightclient",
  "subxt-macro",
  "subxt-metadata",