[Feature Request] Add colSpanAttrs on group.headers slot properties

[ksummerlin]

Problem to solve

It is not possible to determine the column span value needed to create the correct width for the group template unless you copy/paste code from VDataTable.

Proposed solution

The VDataTable correctly calculates the attribute for colspan for the element here:

vuetify/packages/vuetify/src/components/VDataTable/VDataTable.ts

Lines 351 to 354 in e6ebf4f

	const column = this.$createElement('td', {
	staticClass: 'text-start',
	attrs: this.colspanAttrs,
	}, [toggle, `${props.options.groupBy[0]}: ${group}`, remove])

If this.colspanAttrs() could simply be passed into the props for the slot, then the slot template can just use those attributes as well.