Add price rate providers

[timjrobinson]

Add's priceRateProvider field to tokens that then returns a priceRateProviders property on each pool that is the same format as the subgraph - an array of {address, token: {address}}.

The priceRateProvider address is stored in each token rather than in the main pool, but is added to the main pool data when requesting so it looks the same as requesting from the subgraph.

[timjrobinson]

I've added a mutation to sync price rate providers for existing tokens and it appears to work when testing locally. Can run with:

mutation init {
  poolSyncPriceRateProviders
}

[franzns]

I'm waiting for the subgraph to have priceRateProviders also for the gyro pools and yieldFeeExemption as well.

[franzns]

priceRateProviders are now available https://api.thegraph.com/subgraphs/name/balancer-labs/balancer-optimism-v2/graphql?query=%7B%0A%09pools%28%0A++++where%3A+%7B%0A++++++poolType%3A+%22GyroE%22%0A++++++poolTypeVersion%3A+2%0A++++%7D%0A++%29+%7B%0A++++id%0A%09++address%0A++++priceRateProviders+%7B%0A++++++token+%7B%0A++++++++id%0A++++++%7D%0A++++++address%0A++++%7D%0A%09%7D%0A%7D