Revert "FRAME: Create TransactionExtension as a replacement for SignedExtension (#2280)" (#3665)

This PR reverts #2280 which introduced `TransactionExtension` to replace
`SignedExtension`.

As a result of the discussion
[here](https://github.com/paritytech/polkadot-sdk/pull/3623#issuecomment-1986789700),
the changes will be reverted for now with plans to reintroduce the
concept in the future.

---------

Signed-off-by: georgepisaltu <george.pisaltu@parity.io>

docs/sdk/src/reference_docs/signed_extensions.rs

@@ -0,0 +1,79 @@
+//! Signed extensions are, briefly, a means for different chains to extend the "basic" extrinsic
+//! format with custom data that can be checked by the runtime.
+//!
+//! # Example
+//!
+//! Defining a couple of very simple signed extensions looks like the following:
+#![doc = docify::embed!("./src/reference_docs/signed_extensions.rs", signed_extensions_example)]
+
+#[docify::export]
+pub mod signed_extensions_example {
+	use parity_scale_codec::{Decode, Encode};
+	use scale_info::TypeInfo;
+	use sp_runtime::traits::SignedExtension;
+
+	// This doesn't actually check anything, but simply allows
+	// some arbitrary `u32` to be added to the extrinsic payload
+	#[derive(Debug, Encode, Decode, Clone, Eq, PartialEq, TypeInfo)]
+	pub struct AddToPayload(pub u32);
+
+	impl SignedExtension for AddToPayload {
+		const IDENTIFIER: &'static str = "AddToPayload";
+		type AccountId = ();
+		type Call = ();
+		type AdditionalSigned = ();
+		type Pre = ();
+
+		fn additional_signed(
+			&self,
+		) -> Result<
+			Self::AdditionalSigned,
+			sp_runtime::transaction_validity::TransactionValidityError,
+		> {
+			Ok(())
+		}
+
+		fn pre_dispatch(
+			self,
+			_who: &Self::AccountId,
+			_call: &Self::Call,
+			_info: &sp_runtime::traits::DispatchInfoOf<Self::Call>,
+			_len: usize,
+		) -> Result<Self::Pre, sp_runtime::transaction_validity::TransactionValidityError> {
+			Ok(())
+		}
+	}
+
+	// This is the opposite; nothing will be added to the extrinsic payload,
+	// but the AdditionalSigned type (`1234u32`) will be added to the
+	// payload to be signed.
+	#[derive(Debug, Encode, Decode, Clone, Eq, PartialEq, TypeInfo)]
+	pub struct AddToSignaturePayload;
+
+	impl SignedExtension for AddToSignaturePayload {
+		const IDENTIFIER: &'static str = "AddToSignaturePayload";
+		type AccountId = ();
+		type Call = ();
+		type AdditionalSigned = u32;
+		type Pre = ();
+
+		fn additional_signed(
+			&self,
+		) -> Result<
+			Self::AdditionalSigned,
+			sp_runtime::transaction_validity::TransactionValidityError,
+		> {
+			Ok(1234)
+		}
+
+		fn pre_dispatch(
+			self,
+			_who: &Self::AccountId,
+			_call: &Self::Call,
+			_info: &sp_runtime::traits::DispatchInfoOf<Self::Call>,
+			_len: usize,
+		) -> Result<Self::Pre, sp_runtime::transaction_validity::TransactionValidityError> {
+			Ok(())
+		}
+	}
+}