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/utils/mod.rs

@@ -6,6 +6,7 @@
 
 mod account_id;
 pub mod bits;
+mod era;
 mod multi_address;
 mod multi_signature;
 mod static_type;
@@ -16,6 +17,7 @@ use codec::{Compact, Decode, Encode};
 use derivative::Derivative;
 
 pub use account_id::AccountId32;
+pub use era::Era;
 pub use multi_address::MultiAddress;
 pub use multi_signature::MultiSignature;
 pub use static_type::Static;