Add support to read custom fields of OAuth2 introspection response

[ldclakmal]

Description:
Need to support the extraction of custom fields of OAuth2 introspection response [1]. This was addressed for 1.2.x branch with the PR [2].

Sample response:

     HTTP/1.1 200 OK
     Content-Type: application/json

     {
      "active": true,
      "client_id": "l238j323ds-23ij4",
      "username": "jdoe",
      "scope": "read write dolphin",
      "sub": "Z5O3upPC88QrAjx00dis",
      "aud": "https://protected.example.net/resource",
      "iss": "https://server.example.com/",
      "exp": 1419356238,
      "iat": 1419350238,
      "extension_field": "twenty-seven"
     }

[1] https://tools.ietf.org/html/rfc7662#section-2.2
[2] ballerina-platform/ballerina-lang#26335