Add support for private log subscriptions (Pub-Sub API)

[lucassaldanha]

Description

At the moment, the only options for retrieving logs generated by private contracts are using prig_getLogs or private filters.

It would be nice to also offer support for subscribing to private events.

This subscription would be similar to eth_subscribe (we should name it priv_susbscribe).

Acceptance Criteria

• User can use priv_subscribe and priv_unsuibscribe to create subscriptions to private logs.
• This should only support logs subscription.
• The request to create a private logs subscription should be similar to creating a "public" log subscription. The only difference is that the privacyGroupId will be added as parameter index 0.

Definition:

# Subscribe request
{
  "id": 1,
  "method": "priv_subscribe",
  "params": [
    "<privacy_group_id>",
    "logs",
    <filter_parameter_object>
  ]
}

# Unsubscribe request
{
  "id": 1,
  "method": "priv_unsubscribe",
  "params": [
    "<privacy_group_id>",
    "<subscription_id>"
  ]
}

# Subscription notification
{
  "jsonrpc": "2.0",
  "method": "priv_subscription",
  "params": {
    "subscription": "<subscription_id>",
    "privacyGroupId": "<privacy_group_id>",
    "result": <log_entry_object>
  }
}

Example:

# Subscribe request
{
  "id": 1,
  "method": "priv_subscribe",
  "params": [
     "OGD/1dkDZWb9VqgDfElovjYMDAcSiRUiB6fLtFRmugU=",
    "logs",
    {
      "address": "0x8320fe7702b96808f7bbc0d4a888ed1468216cfd",
      "topics": [
        "0xd78a0cb8bb633d06981248b816e7bd33c2a35a6089241d099fa519e361cab902"
      ]
    }
  ]
}

# Unsubscribe request
{
  "id": 1,
  "method": "priv_unsubscribe",
  "params": [
    "OGD/1dkDZWb9VqgDfElovjYMDAcSiRUiB6fLtFRmugU=",
    "0xcd0c3e8af590364c09d0fa6a1210faf5"
  ]
}

# Subscription notification
{
  "jsonrpc": "2.0",
  "method": "priv_subscription",
  "params": {
    "subscription": "0x1",
    "privacyGroupId": "awEvxROVE0ydIgZyVFOfjvZKxaUwtOcWha/CJFHJ4Lw=",
    "result": {
      "logIndex": "0x0",
      "removed": false,
      "blockNumber": "0x2de",
      "blockHash": "0xe4657882a821af048bafbe64a6a0d40e61229c2ad1ef70bc9627e3adf0c941f7",
      "transactionHash": "0xd3e7c320a3d3a8a28fa722611b12c52452dc1ed9e6b0400f7421628f1360b064",
      "transactionIndex": "0x0",
      "address": "0xd9104a5e123c0a026e35eaf7b500bd026921dbde",
      "data": "0x",
      "topics": [
        "0x85bea11d86cefb165374e0f727bacf21dc2f4ea816493981ecf72dcfb212a410",
        "0x0000000000000000000000000000000000000000000000000000000000000002"
      ]
    }
  }
}