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/src/blocks/extrinsic_types.rs

@@ -13,7 +13,7 @@ use crate::{
 };
 
 use crate::config::signed_extensions::{
-    ChargeAssetTxPayment, ChargeTransactionPayment, CheckNonce,
+    ChargeAssetTxPayment, ChargeTransactionPayment, CheckNonce, SkipCheckIfFeeless,
 };
 use crate::config::SignedExtension;
 use crate::dynamic::DecodedValue;
@@ -685,7 +685,7 @@ impl<'a, T: Config> ExtrinsicSignedExtensions<'a, T> {
     ///
     /// Returns `None` if  `tip` was not found or decoding failed.
     pub fn tip(&self) -> Option<u128> {
-        // Note: the overhead of iterating twice should be negligible.
+        // Note: the overhead of iterating multiple time should be negligible.
         self.find::<ChargeTransactionPayment>()
             .ok()
             .flatten()
@@ -696,6 +696,12 @@ impl<'a, T: Config> ExtrinsicSignedExtensions<'a, T> {
                     .flatten()
                     .map(|e| e.tip())
             })
+            .or_else(|| {
+                self.find::<SkipCheckIfFeeless<T, ChargeAssetTxPayment<T>>>()
+                    .ok()
+                    .flatten()
+                    .map(|skip_check| skip_check.inner_signed_extension().tip())
+            })
     }
 
     /// The nonce of the account that submitted the extrinsic, extracted from the CheckNonce signed extension.