Add ConstFeeMultiplier to the transaction payment pallet (#12222)

* fix: FeeMultiplierUpdate

* fix: cargo fmt

* fix: rustdoc

* Revert "fix: rustdoc"

This reverts commit 96b6ad80a4cd4d856cf5a830889858c4dd4c385b.

* Revert "fix: cargo fmt"

This reverts commit 13016527bdbc53d9484642d6b52430550c0efc55.

* Revert "fix: FeeMultiplierUpdate"

This reverts commit 2cbddd0b85e0293d0eeda859807ddf70cee29067.

* feat: add ConstFeeMultiplier

* fix: use cConstFeeMultiplier in the template node

substrate/frame/transaction-payment/src/lib.rs

@@ -226,6 +226,30 @@ where
 	}
 }
 
+/// A struct to make the fee multiplier a constant
+pub struct ConstFeeMultiplier<M: Get<Multiplier>>(sp_std::marker::PhantomData<M>);
+
+impl<M: Get<Multiplier>> MultiplierUpdate for ConstFeeMultiplier<M> {
+	fn min() -> Multiplier {
+		M::get()
+	}
+	fn target() -> Perquintill {
+		Default::default()
+	}
+	fn variability() -> Multiplier {
+		Default::default()
+	}
+}
+
+impl<M> Convert<Multiplier, Multiplier> for ConstFeeMultiplier<M>
+where
+	M: Get<Multiplier>,
+{
+	fn convert(_previous: Multiplier) -> Multiplier {
+		Self::min()
+	}
+}
+
 /// Storage releases of the pallet.
 #[derive(Encode, Decode, Clone, Copy, PartialEq, Eq, RuntimeDebug, TypeInfo, MaxEncodedLen)]
 enum Releases {
@@ -371,7 +395,8 @@ pub mod pallet {
 			// add 1 percent;
 			let addition = target / 100;
 			if addition == Weight::zero() {
-				// this is most likely because in a test setup we set everything to ().
+				// this is most likely because in a test setup we set everything to ()
+				// or to `ConstFeeMultiplier`.
 				return
 			}