Direct-approval router

[jubeira]

Some integrators can't use permit2, and would rather granting approvals to the routers directly.

One option would be to reuse existing code, add an immutable flag that can be configured during construction time, and use permit2 or direct transferFroms depending on it.

[jubeira]

Let's start with a transfer-first router, only for swaps exact in.

The aggregator pays upfront, then calls the router and gets the tokens owed. No approvals needed at all.

Use #727 as a base.