[api] Add Savers Capacity Fields to Pool Endpoints

Adds savers capacity QOL fields to /pool endpoints, addressing issue #1983 (closed)

verified /pool and /pools response in mocknet

 {
    "asset": "ETH.ETH",
    ...
    "savers_fill_bps": "0",
    "savers_capacity_remaining": "927594915",
    ...
  }