add radar layer custom color support

[penkzhou]

PR Checklist:

• [x] I have tested this extensively and it does not break any existing behavior.
• [x] I have added/updated examples and tests for any new behavior.
• [x] If this is a significant change, an issue has already been created where the problem / solution was discussed: [N/A, or add link to issue here]

PR Description

before the pr, radar chart cannot custom the chart layer background, like this:

before the pr, radar chart can custom the chart layer background by a new api, like this:

radar chart can custom the chart layer background by a new api

reference by this:https://stackoverflow.com/questions/71331675/how-to-fill-color-layer-yaxis-in-mpandroidchart-radar-chart/