Allow setting "control-cols" on horizontal form group

[Trainmaster]

Is your feature request related to a problem? Please describe...

I'm creating forms using the <b-form-group> element wrapper. While the label width is adjustable via the label-cols parameter, there is no parameter for the control element width. Setting the control element width via e.g. class="col-3" results in two nested divs with class col and thus unwanted padding left/right.

Describe the solution you'd like

Make the control element width adjustable on the <b-form-group> via control-cols.

Describe alternatives you've considered

• Adding class="col-3 px-0" to the control element
• Using utility classes on the control element w-50
• Setting the width via inline-styles