Add tracking of volume in pools over time

### Detailed Description
As a user, i want to be aware of the volume that flows through a given existing XYK/LBP pool at any given block (time) in the past.

### Context
This feature will be used to display the historical volume graph on the UI.

### Possible Implementation

Example, please don't follow 1:1

```
type VolumeOverTime {
id: ID! // same as in #7
// tracking the overall (buy + sell) liquidity for individual assets
// will allow us to display a more sophisticated UI
assetA: BigInt!,
assetB: BigInt!
}
```