Make ExtrinsicParams more flexible, and introduce signed extensions (#1107)

* WIP new SignedExtensions

* Integrate new signex extension stuff, update docs, remove Static wrapper

* remove impl Into in tx_client to avoid type inference annoyances

* clippy and fix example

* Fix book links

* clippy

* book tweaks

* fmt: remove spaces

Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>

* re-expose Era in utils, and tweak wasm-example

* clippy; remove useless conversion

---------

Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>

subxt/src/error/mod.rs

@@ -17,6 +17,7 @@ pub use dispatch_error::{
 };
 
 // Re-expose the errors we use from other crates here:
+pub use crate::config::ExtrinsicParamsError;
 pub use crate::metadata::Metadata;
 pub use scale_decode::Error as DecodeError;
 pub use scale_encode::Error as EncodeError;
@@ -58,6 +59,9 @@ pub enum Error {
     /// Transaction progress error.
     #[error("Transaction error: {0}")]
     Transaction(#[from] TransactionError),
+    /// Error constructing the appropriate extrinsic params.
+    #[error("{0}")]
+    ExtrinsicParams(#[from] ExtrinsicParamsError),
     /// Block related error.
     #[error("Block error: {0}")]
     Block(#[from] BlockError),
@@ -88,6 +92,12 @@ impl From<String> for Error {
     }
 }
 
+impl From<std::convert::Infallible> for Error {
+    fn from(value: std::convert::Infallible) -> Self {
+        match value {}
+    }
+}
+
 /// An RPC error. Since we are generic over the RPC client that is used,
 /// the error is boxed and could be casted.
 #[derive(Debug, thiserror::Error)]