feat(app): 🎣 add missing Dex component hook (#4247)

see #3739, and #4246.

while wiring up a pair of missing component hooks for the `Sct`
component, i found another (noöp) hook that we do not include in the
`App`'s control flow.

this isn't currently a bug, since `<Dex as Component>::begin_block()` is
a noöps but could easily lead to a surprising outcome in the future, if
more logic was introduced to that trait method.

#### checklist before requesting a review

- [x] if this code contains consensus-breaking changes, i have added the
"consensus-breaking" label. otherwise, i declare my belief that there
are not consensus-breaking changes, for the following reason:

> while this **does** change the control flow of consensus-critical
logic, this does **not**
change the execution of incoming data. the hooks introduced here are
noöps.

crates/core/app/src/app/mod.rs

@@ -257,6 +257,7 @@ impl App {
             begin_block,
         )
         .await;
+        Dex::begin_block(&mut arc_state_tx, begin_block).await;
         CommunityPool::begin_block(&mut arc_state_tx, begin_block).await;
         Governance::begin_block(&mut arc_state_tx, begin_block).await;
         Staking::begin_block(&mut arc_state_tx, begin_block).await;