unpack from vector should be implemented in hive fork manager

get_witnesses() method of haf block explorer will use haf data.

In order to derive feed_price, url, signing_key, block_size from witness_set_properties_operation, stored procedure hive.unpack_from_vector(_vector) should take single text parameter and decode it.

Example: the hbd_exchange_rate value, "15020000000000000353424400000000e80300000000000003535445454d0000":

{
  "type": "witness_set_properties_operation",
  "value": {
    "owner": "ocd-witness",
    "props": [
      [
        "hbd_exchange_rate",
        "15020000000000000353424400000000e80300000000000003535445454d0000"
      ],
      [
        "key",
        "039e1eb02f05fad2ddec7d10743e1566f927fa89fef3fbab255fa1ea276053ce3b"
      ]
    ],
    "extensions": []
  }
}

should be returned as decoded price object with nested base and quote, i.e. same JSON format as exchange_rate object of feed_publish_operation

{
  "type": "feed_publish_operation",
  "value": {
    "publisher": "pharesim",
    "exchange_rate": {
      "base": {
        "amount": "540",
        "precision": 3,
        "nai": "@@000000013"
      },
      "quote": {
        "amount": "1300",
        "precision": 3,
        "nai": "@@000000021"
      }
    }
  }
}

This should work for all vectors in props of witness_set_properties_operation, regardless of their name

{
  "type": "witness_set_properties_operation",
  "value": {
    "owner": "chisdealhd",
    "props": [
      [
        "account_creation_fee",
        "b80b00000000000003535445454d0000"
      ],
      [
        "account_subsidy_budget",
        "1d030000"
      ],
      [
        "account_subsidy_decay",
        "b94c0500"
      ],
      [
        "hbd_interest_rate",
        "dc05"
      ],
      [
        "key",
        "02d912ebc6358fe5b7d86964b9714b5e4d04c2d537c6f6305a2cdfcc22a0b0dc47"
      ],
      [
        "maximum_block_size",
        "00000100"
      ],
      [
        "new_signing_key",
        "000000000000000000000000000000000000000000000000000000000000000000"
      ],
      [
        "url",
        "5b68747470733a2f2f7065616b642e636f6d2f686976652d3131313131312f4073686d6f6f676c656f73756b616d692f68656c702d737570706f72742d636869736465616c6864732d686976652d7769746e6573732d736572766572"
      ]
    ],
    "extensions": []
  }
}