Add `saversAPR` to `/pools` endpoint

add saversAPR based on formula below:

if the period is longer than when savers initiated, Midgard will show 0

(24hrs as period param)

unitsBefore = saver's units 24hrs bucket before than current time (it might be different than `unitsNow` due to stake, and withdraws)
unitsNow = saver's units now
depthBefore = saver's depth 24hrs bucket before now
depthNow = saver's depth now
beforeGrowthIndex = depthBefore / unitsBefore
nowGrowthIndex = depthNow / unitsNow
----
saversAPR = (nowGrowthIndex - beforeGrowthIndex) * blocksPerYear/24hrsBlockCount

{
	"annualPercentageRate": "0.07447201962776606",
	"asset": "BTC.BTC",
	"assetDepth": "118241681",
	"assetPrice": "14608.295471340602",
	"assetPriceUSD": "17660.720513651784",
	"liquidityUnits": "1223159714705",
	"nativeDecimal": "8",
	"poolAPY": "0.07447201962776606",
	"runeDepth": "1727309413076",
	"saversDepth": "100901",
	"saversUnits": "100853",
	"status": "available",
	"synthSupply": "17592752",
	"synthUnits": "98308216680",
	"units": "1321467931385",
	"volume24h": "0"
}

{
  "annualPercentageRate": "0.0532067787486169",
  "asset": "BTC.BTC",
  "assetDepth": "118246541",
  "assetPrice": "14544.627383104593",
  "assetPriceUSD": "17660.720513651784",
  "liquidityUnits": "1223159714705",
  "nativeDecimal": "8",
  "poolAPY": "0.0532067787486169",
  "runeDepth": "1719851878186",
  "saversAPR": "0.3219128408886943", // adds savers APR based on the period param (default 30d)
  "saversDepth": "100901",
  "saversUnits": "100853",
  "status": "available",
  "synthSupply": "17592752",
  "synthUnits": "98303851424",
  "units": "1321463566129",
  "volume24h": "176960415464"
}