x/leverage: frontend summary query

[toteki]

Frontend team is considering the possibility of combined queries that streamline some of the traffic on the frontend.

If implemented directly like the other queries (rather than in some middleware), it would likely resemble:

https://api.poke.umeemania-1.network.umee.cc/umee/leverage/v1beta1/front_end_summary?denom=ibc/<denom>

{
  "exchange_rates": [
    {
      "denom": "ATOM",
      "amount": "14.001388914790883104"
    }
  ]
"lend_APY": "0.023353603293602098", // lend APY
"borrow_APY": "0.087485408130324650", // borrow APY
"market_size_usd": "16840428.406458300000000000", //marketSize USD
"amount": "64266663653122" // availableBorrow
}

Original issue: https://github.com/umee-network/umee-v1-client/issues/434