Gets the value stored for this field on the provided issue.
issue - identifies the issue that will contain the value for this field.
the value stored on this issue for this field, null if not applicable.
Defines if a field determines itself as renderable. This is needed because of the way that customfields
are implemented. Since all the real work is done in CustomFieldTypes, if we want to treat system fields
and custom fields the same way in the renderers then all custom fields must implement this interface. We
therefore provide this method so that the CustomFieldImpl can delegate to the CustomFieldTypes to determine
if a given custom field is renderable.