How Customizable should controls be?

[fallaciousreasoning]

It's common to have slightly different usecases for common controls, where the default styling is almost but not quite right. In these cases, it's useful for consumers to be able to customize parts of the control, so they get the benefits of the built in Leo control (including accessibility, keyboard control, default styling, theme support).

An example given recently was customizing the label on our Checkbox, where we want to be able to build the following layout:

This is not currently possible.

Possible approaches:

1. Expose a --leo-checkbox-direction which supports different flex directions?
2. Expose a css ::part?
3. Expose a bunch of properties from the control?

@petemill @aguscruiz @AlanBreck