ref(chain): move keychain::ChangeSet into txout_index.rs

We plan to record `Descriptor` additions into persistence. Hence, we
need to add `Descriptor`s to the changeset. This depends on
`miniscript`. Moving this into `txout_index.rs` makes sense as this is
consistent with all the other files. The only reason why this wasn't
this way before, is because the changeset didn't need miniscript.

Co-Authored-By: Daniela Brozzoni <danielabrozzoni@protonmail.com>

crates/chain/src/indexed_tx_graph.rs

@@ -320,6 +320,7 @@ impl<A, IA: Default> From<tx_graph::ChangeSet<A>> for ChangeSet<A, IA> {
     }
 }
 
+#[cfg(feature = "miniscript")]
 impl<A, K> From<keychain::ChangeSet<K>> for ChangeSet<A, keychain::ChangeSet<K>> {
     fn from(indexer: keychain::ChangeSet<K>) -> Self {
         Self {