Support decoding signed extensions (#1209)

* skeleton commit

* signed extension decoding

* fix some minor things

* make api more similar to Extrinsics

* defer decoding of signed extensions

* fix byte slices

* add test for nonce signed extension

* adjust test and extend for tip

* clippy

* support both  ChargeTransactionPayment and ChargeAssetTxPayment

* address PR comments

* Extend lifetimes, expose pub structs, remove as_type

* add signed extensions to block subscribing example

* make example less ugly

---------

Co-authored-by: James Wilson <james@jsdw.me>

subxt/examples/blocks_subscribing.rs

@@ -48,6 +48,16 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
                 println!("        {pallet_name}_{event_name}");
                 println!("          {}", event_values);
             }
+
+            println!("      Signed Extensions:");
+            if let Some(signed_extensions) = ext.signed_extensions() {
+                for signed_extension in signed_extensions.iter() {
+                    let signed_extension = signed_extension?;
+                    let name = signed_extension.name();
+                    let value = signed_extension.value()?.to_string();
+                    println!("        {name}: {value}");
+                }
+            }
         }
     }