chore: Rename address_for_approvals -> router_address

When will this not be the router address that we use for approvals? It will always be the router address. address_for_approvals sounds to me very vague (which approvals?).

src/encoding/models.rs

@@ -76,6 +76,5 @@ pub struct Transaction {
 pub struct EncodingContext {
     pub receiver: Bytes,
     pub exact_out: bool,
-    // TODO I don't like this - what does this mean?
-    pub address_for_approvals: Bytes,
+    pub router_address: Bytes,
 }