fix: limit amm swap route length

[sander2]

Limits the route length in swap_exact_token_for_tokens_through_stable_pool. Longer term we might want to make the weight parametric over the route length. A slight complication is that to determine the number of swaps, you need to iterate over the route vector, so you'd be spending some computation prior to execution, which is not ideal. We could add an additional parameter containing the number of swaps to prevent that. But anyway for this fix will suffice

Addresses https://github.com/interlay/srlabs-audit/issues/10