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/error/mod.rs

@@ -144,23 +144,19 @@ impl BlockError {
 #[derive(Clone, Debug, Eq, thiserror::Error, PartialEq)]
 #[non_exhaustive]
 pub enum TransactionError {
-    /// The finality subscription expired (after ~512 blocks we give up if the
-    /// block hasn't yet been finalized).
-    #[error("The finality subscription expired")]
-    FinalityTimeout,
     /// The block hash that the transaction was added to could not be found.
     /// This is probably because the block was retracted before being finalized.
     #[error("The block containing the transaction can no longer be found (perhaps it was on a non-finalized fork?)")]
     BlockNotFound,
-    /// The transaction was deemed invalid in the current chain state.
-    #[error("The transaction is no longer valid")]
-    Invalid,
-    /// The transaction was replaced by a transaction with the same (sender, nonce) pair but with higher priority
-    #[error("The transaction was replaced by a transaction with the same (sender, nonce) pair but with higher priority.")]
-    Usurped,
-    /// The transaction was dropped because of some limit
-    #[error("The transaction was dropped from the pool because of a limit.")]
-    Dropped,
+    /// An error happened on the node that the transaction was submitted to.
+    #[error("Error handling transaction: {0}")]
+    Error(String),
+    /// The transaction was deemed invalid.
+    #[error("The transaction is not valid: {0}")]
+    Invalid(String),
+    /// The transaction was dropped.
+    #[error("The transaction was dropped: {0}")]
+    Dropped(String),
 }
 
 /// Something went wrong trying to encode a storage address.