Add an extra_constant to pallet-treasury (#7918)

Aligned with #7206, this PR exposes `pallet-treasury`'s pot account as
an extra constant. Users can this way for instance donate to the chain's
treasury account, or founders can easily spot it in case they require to
fund it.

In any case this greatly reduces errors and headaches trying to figure
out what's the pot's address.

---------

Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>

prdoc/pr_7918.prdoc

+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: "Add an extra_constant to pallet-treasury"
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      - Allows to query pallet-treasury's pot account via extra constant.
+
+crates:
+  - name: pallet-treasury
+    bump: minor

substrate/frame/treasury/src/lib.rs

@@ -289,6 +289,14 @@ pub mod pallet {
 		type BlockNumberProvider: BlockNumberProvider;
 	}
 
+	#[pallet::extra_constants]
+	impl<T: Config<I>, I: 'static> Pallet<T, I> {
+		/// Gets this pallet's derived pot account.
+		fn pot_account() -> T::AccountId {
+			Self::account_id()
+		}
+	}
+
 	/// DEPRECATED: associated with `spend_local` call and will be removed in May 2025.
 	/// Refer to <https://github.com/paritytech/polkadot-sdk/pull/5961> for migration to `spend`.
 	///