Allow specifying groupable columns in ui.table

Most of the time, I don't want to group by all columns. I want the dropdown list to show a selected few choices.

right now, `groupable = True` includes all the columns.

#### Option 1:
If we change the signature to `Optional[Union[bool | List[str]]] = None`:
- `False`: no goupby
- `True`: groupby all columns
- `['col1', 'col3']`: grouping dropdown shows only `col1` and `col3`

#### Option 2:
If we change the signature to `Optional[List[str]] = None`:
- `None` or `[]`: treat as `groupable = False`
- `['col1', 'col3']`: grouping dropdown shows only `col1` and `col3`
- rename `groupable` to `groupby`