x/gamm: Make gas cost per swap a param

[rrrliu]

Background

Follow up to #1903 (comment), make this fixed gas fee into a pool-level param for more customizability.

Right now we use a BalancerGasFeeForSwap fixed constant, we'll need to update that logic when we swap.

Suggested Design

Add a new field to PoolParams for balancer and stableswap pools, use this field to consume gas during applySwap.

Acceptance Criteria

• PoolParams struct updated, prepare for contract migration
• Unit tests in pool_test.go and updated tests in swap_test.go

[alexanderbez]

Per chain-dev call, we decided to keep this as a static-hard coded value.