Allow custom fields of type "Text Field" to be used as the Statistic Type for the Issue Statistics gadget

Problem Definition

Currently you can not select a text field type for use in the statistic type of the pie chart. However fields of type such as labels, versions, and select fields are all able to be used here.

Suggested Solution

It would be helpful if we could use a text field in the same way we use a select field for this statistic type.

Why this is important

In many instances of Jira, data has already been created in a field type of text, but then this data cannot be used to show issue statistics data. Converting this data into a select field is rather tedious to accomplish in Jira. It would be helpful if the gadget could simply use those field types instead.