Remove PoolAsset and manage corresponding fields in Balancer Params

[mattverse]

Implementing gamm module generalization for different pool-models, we should move poolAsset in pool.proto into balancerParams or balancerPool. Token and token weights should be managed within the balancerPool package for better generalization of the gamm module.

[mconcat]

We may need to move pool weights into the balancer pool model, but not the pool assets, for managing the assets on the keeper side. Ref #131