title: Filter Query by any field, not just defined available filters
body: <p>PS: my GraphQL skills are pretty basic so sorry for any incorrect use of words and terms</p>
  <p>I want to achieve filtering on  the <strong>code</strong> field highlighted below</p>
  <p>(transactions --&gt; edges --&gt; node --&gt; header --&gt; transactionSource --&gt; code = &quot;something&quot;</p>
  <pre><code>    {
    transactions(last: 10) {
      edges {
        node {
          amount
          periodId
          header {
            owner {
              owner {
                code
                description
                dbId
                ownerDbId
                ownerCode
              }
            }
            transactionSource {
              code
            }
          }
        }
      }
      pageInfo {
        hasNextPage
      }
    }
  }
  </code></pre>
  <p>The client i'm working with have defined a list of filtering options, which I can successfully filter on, but when I try to filter on the <strong>code</strong> field I get the following result:</p>
  <pre><code>{
    &quot;errors&quot;: [
      {
        &quot;message&quot;: &quot;Argument 'filter' has invalid value. In field 'code': Unknown field.&quot;,
        &quot;locations&quot;: [
          {
            &quot;line&quot;: 2,
            &quot;column&quot;: 26
          }
        ],
        &quot;extensions&quot;: {
          &quot;code&quot;: &quot;ARGUMENTS_OF_CORRECT_TYPE&quot;,
          &quot;codes&quot;: [
            &quot;ARGUMENTS_OF_CORRECT_TYPE&quot;
          ],
          &quot;number&quot;: &quot;5.6.1&quot;
        }
      }
    ]
  }
  </code></pre>
  <p>I presume this is because of me not knowing exactly how to set up the filter correctly.
  Is there a way to filter on any field or do I need to talk to the guys maintaining the client and ask them nicely to make the <strong>code</strong> field available for filtering?</p>
  <p>Thanks</p>