mgr/dashboard: support custom executing template in table cells

mgr/dashboard: support custom executing template in table cells

Now we can modify the style of a table that has an executing state
by providing a custom config when declaring columns: e.g.,

```
    this.columns = [
      {
        prop: 'id',
        name: $localize`ID`,
        flexGrow: 1,
        cellTransformation: CellTemplate.executing,
        customTemplateConfig: {
          valueClass: 'a',
          executingClass: 'b'
        }
      },
```

Which makes the cell value render with class `a` and the executing state
(e.g. (updating)) render with class `b`.

Signed-off-by: Kiefer Chang <kiefer.chang@suse.com>