Feature: custom crosshair labels formatter

[dan12411]

Description

Current crosshair labels' format is followed by scale's formatter. The PR is try to give custom formatter for crosshair labels.

At first, fix the label value.(should denormalize value before invert) - 6a8b10a

And Then add the formatter property - 326511c

For example,

Original format

Custom format

                   crosshair: CrosshairGuide(
                     ...
                     formatter: [
                       (date) => DateFormat('MM/dd').format(DateTime.parse(date as String)),
                       (volume) => (volume as num).toStringAsFixed(0),
                     ],
                   ),