Assets API for Statemint

[joepetrowski]

GET assets/{assetId}/asset-info

{
  at: { ... },
  AssetInfo: https://github.com/paritytech/substrate/blob/069917b/frame/assets/src/lib.rs#L186
  AssetMetadata: https://github.com/paritytech/substrate/blob/069917b/frame/assets/src/lib.rs#L258
}

GET accounts/{accountId}/assets-balances?asset=assetId

WITH param
{
  at: { ... },
  balance: Balance,
  isFrozen: bool
 // https://github.com/paritytech/substrate/blob/069917b/frame/assets/src/lib.rs#L248
}

WITHOUT param
{
  at: { ... },
  assets: [{ assetId: int, balance: Balance, isFrozen: bool}]
}

GET accounts/{accountId}/asset-approvals?asset=assetId&delegate=accountId

{
  at: { ... },
  amount: Balance, // amount approved for transfer
  deposit: Balance // amount owner has on deposit to store the approval
}

[TarikGul]

@joepetrowski Picked this up again, and have now made it priority. Expect to see some heavy progress on this in the next few days.