Closes #91: Add a GET endpoint to list filter subscriptions

[fllaca]

Closes #91 . First approach to have an endpoint to list the existing filter subscriptions.

Output:

➜  eventeum git:(master) ✗ curl -XGET -H "Content-Type: application/json" localhost:8060/api/rest/v1/event-filter | jq
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   670    0   670    0     0  44666      0 --:--:-- --:--:-- --:--:-- 44666
[
  {
    "id": "my-custom-event",
    "contractAddress": "0xd6dE21390c5D2045A4f8477133bb9a23ea9c3F88",
    "node": "default",
    "eventSpecification": {
      "eventName": "Mint",
      "indexedParameterDefinitions": [
        {
          "position": 0,
          "type": "ADDRESS"
        }
      ],
      "nonIndexedParameterDefinitions": [
        {
          "position": 1,
          "type": "STRING"
        },
        {
          "position": 2,
          "type": "UINT256"
        }
      ]
    },
    "startBlock": 0
  },
  {
    "id": "a17fd147-1c36-4759-b2a4-024d2ac9f46f",
    "contractAddress": "0xd6dE21390c5A2045A4b8477133bb9a23ea9c3F66",
    "node": "default",
    "eventSpecification": {
      "eventName": "Mint",
      "indexedParameterDefinitions": [
        {
          "position": 0,
          "type": "ADDRESS"
        }
      ],
      "nonIndexedParameterDefinitions": [
        {
          "position": 1,
          "type": "STRING"
        },
        {
          "position": 2,
          "type": "UINT256"
        }
      ]
    },
    "startBlock": 0
  }
]

[craigwilliams84]

Thanks Fernando! Could you also add some documentation for the endpoint in the README? Also, a basic few integration tests (in the server project) could be useful.