fix: add nominations in staking-info endpoint

[Imod7]

Description

Closes #1446

This PR adds the field nominations in the staking-info endpoint's response which was missing. This information is retrieved from the api.query.staking.nominators call.

Response (before the fix)

When connected to the Polkadot chain, the following request

http://127.0.0.1:8080/accounts/16GMHo9HZv8CcJy4WLoMaU9qusgzx2wxKDLbXStEBvt5274B/staking-info?at=20300500

results in the response below:

{
  "at": {
    "hash": "0xfee9a90faaaa0e29f3feb53360bf8527ffdfa295267602b35ed8990bf01d5493",
    "height": "20300500"
  },
  "controller": "16eM1npMwKzpGy48NDna1jC6P71S783wjpbdeKT8RgzQx8Jd",
  "rewardDestination": {
    "account": "13FzGLWoueKvUqFePiJgvFYWhH5KckHGtVBXvAX7SBtVZbXu"
  },
  "numSlashingSpans": "0",
  "staking": {
    ...
    "claimedRewards": [
      "104",
       ...
      "187"
    ]
  }
}

Response (after the fix)

{
  "at": {
    "hash": "0xfee9a90faaaa0e29f3feb53360bf8527ffdfa295267602b35ed8990bf01d5493",
    "height": "20300500"
  },
  "controller": "16eM1npMwKzpGy48NDna1jC6P71S783wjpbdeKT8RgzQx8Jd",
  "rewardDestination": {
    "account": "13FzGLWoueKvUqFePiJgvFYWhH5KckHGtVBXvAX7SBtVZbXu"
  },
  "numSlashingSpans": "0",
  "nominations": {
    "targets": [
      "16CdHjb4nxVwF6uwmPm6A29pc4ubnLiY7UqasMxt7cT9BcoK",
      "12YUDdiZCHE47WqR6etGD4gfdpSUhrRFvASrVhtc1RcbLva8",
      "1AvwyrRcECEWUnkRoqfcyT2oA11uBT66aS4p127Mbs4ocjq",
      "1JoBYyPoUdsuU7vZi3KgQAaQYn6WhKqUDXRDmsaJ8Zgxr4T",
      "13VS9jM2pkKNwd8LFpQNNRPBKUonU9jzGZ5fTEZiuB4nYF6Y"
    ],
    "submittedIn": "1409",
    "suppressed": false
  },
  "staking": {
    ...
    "claimedRewards": [
      "104",
      ....
      "187"
    ]
  }
}

The result in nominations was also cross checked with pjs-apps call staking.nominators for the same account and block (block hash : 0xfee9a90faaaa0e29f3feb53360bf8527ffdfa295267602b35ed8990bf01d5493)

Todos

• Docs were not updated since the nominations information was already included
• Update tests

[IkerAlus]

LGTM!