[Example] Implement custom field transformer to format data in list

[niklasnatter]

What's in this PR?

This PR demonstrates how to implement a custom field transformer, which can be used to format data inside of lists. The example registers a simple colored_text field transformer that renders the data of the column with a random color.

To apply the changes, the administration frontend application needs to be rebuilt by executing bin/console sulu:admin:update-build.