Allow user to override draw function in GridImageryProvider

This came up in https://github.com/AnalyticalGraphicsInc/cesium/pull/8280. The idea is to make it easy for the developer to create effects like this hexagonal grid by allowing them to override the draw function in the `GridImageryProvider`:

```
Cesium.Viewer('container', {
imageryProvider: Cesium.GridImageryProvider({
cells: 3,
drawGrid: function(context) {
// `context` is the canvas 2d context,
// you can use it to draw what you want.
}
})
});
```


Is this something you would use in your application? If so, please :+1: or describe your use case in this issue. Right now this is still possible to do by creating a custom imagery provider based off of the `GridImageryProvider` in your application.

Locate the GridImageryProvider entry point and inspect how its canvas drawing currently works, then review the linked pull request for the motivating hexagonal-grid example. Done means developers can supply a drawGrid callback that receives the canvas 2D context and the existing grid behavior remains available when no callback is provided; add or update the relevant tests if the project has coverage for this provider.

Written by the indexing model from the issue text.