config: Add SkipCheckIfFeeless signed extension (#1264)

* config: Add `SkipCheckIfFeeless` signed extension

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* config: Add extra extension to the default params

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* examples: Adjust signed extension example

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* config: Extend SkipCheckIfFeeless with inner signed extension

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* config: Configure SkipCheck with inner signed extension params

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* config: Implement Deafult for SkipCheckIfFeelessParams with Option

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* examples: Fix example with proper extension

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* config: Extend <T as Config>::AssetId with EncodeAsType and Clone

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* config: Add SkipCheck with AssetTx

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* config: Encode as type from metadata the inner signed extensions

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Adjust examples

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* blocks: Use `SkipCheckIfFeeless` for decoding the tip of extensions

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* config: Decode `SkipCheckIfFeeless` with `Self`

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* tests: Adjust testing

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* config: Descriptive errors for building `SkipCheckIfFeeless`

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* config: Add docs for extra error types

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* subxt: Add extra derives to signed extensions

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* config: Use `Default::default` to simplify type init

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

subxt/examples/setup_config_signed_extension.rs

@@ -1,4 +1,5 @@
 use codec::Encode;
+use scale_encode::EncodeAsType;
 use subxt::client::OfflineClientT;
 use subxt::config::signed_extensions;
 use subxt::config::{
@@ -11,6 +12,7 @@ pub mod runtime {}
 
 // We don't need to construct this at runtime,
 // so an empty enum is appropriate:
+#[derive(EncodeAsType)]
 pub enum CustomConfig {}
 
 impl Config for CustomConfig {
@@ -32,6 +34,10 @@ impl Config for CustomConfig {
             signed_extensions::CheckMortality<Self>,
             signed_extensions::ChargeAssetTxPayment<Self>,
             signed_extensions::ChargeTransactionPayment,
+            signed_extensions::SkipCheckIfFeeless<
+                Self,
+                signed_extensions::ChargeAssetTxPayment<Self>,
+            >,
             // And add a new one of our own:
             CustomSignedExtension,
         ),
@@ -81,8 +87,8 @@ impl ExtrinsicParamsEncoder for CustomSignedExtension {
 pub fn custom(
     params: DefaultExtrinsicParamsBuilder<CustomConfig>,
 ) -> <<CustomConfig as Config>::ExtrinsicParams as ExtrinsicParams<CustomConfig>>::OtherParams {
-    let (a, b, c, d, e, f, g) = params.build();
-    (a, b, c, d, e, f, g, ())
+    let (a, b, c, d, e, f, g, h) = params.build();
+    (a, b, c, d, e, f, g, h, ())
 }
 
 #[tokio::main]