Feature request: expose `outer.radius` in `coord_radial()`

[Yunuuuu]

coord_radial() currently has outer.radius hardcoded to 0.8 (0.4 = 0.8/2), preventing users from adjusting the large spacing described in: #6284.

If this looks good, I can submit a pull request.

[teunbrand]

I think this'll complicate axis placement, but if this can be done without negatively affecting these at various radii, I'm not against this. However, I think this would be best parameterised as a margin/padding/spacing, rather than giving direct control over the radii.

[Yunuuuu]

That makes sense. If we parameterize this as a margin/padding/spacing, would it conflict with the plot margin defined in the theme? Or would it be appropriate to make this parameter an element within the theme itself?

[teunbrand]

I don't think the theme is available when data is being transformed, so I don't think a theme setting would make a lot of sense from a plumbing perspective. There'd be a slight semantic overlap between the spacing and the plot.margin, but I'm not too worried about this.