feat: ApprovalsManager trait

Make Permit2 and TokenApprovalsManager implement it.
This way in the RouternEncoder we can use either one (I'm not exactly sure what this would mean in the contract though)

I'm not sure I like this generalisation. The TokenApprovalsManager was made with a different purpose: to approve token allowances for the pools (like balancer and curve) for our router

src/encoding/mod.rs

@@ -1,6 +1,5 @@
-mod approvals_manager;
+mod approvals;
 mod models;
-mod permit2;
 mod router_encoder;
 mod strategy_encoder;
 mod strategy_selector;