[ButtonGroup] Allow override of the variant prop

[nate-vukovich]

• I have followed (at least) the PR section of the contributing guide.

Closes #16918

[mui-pr-bot]

Details of bundle changes.

Comparing: 5dc2402...effe473

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	+0.01% 🔺	+0.01% 🔺	329,712	329,729	90,214	90,219
@material-ui/core/Paper	0.00%	0.00%	68,684	68,684	20,469	20,469
@material-ui/core/Paper.esm	0.00%	0.00%	62,058	62,058	19,209	19,209
@material-ui/core/Popper	0.00%	0.00%	29,185	29,185	10,434	10,434
@material-ui/core/Textarea	0.00%	0.00%	5,766	5,766	2,372	2,372
@material-ui/core/TrapFocus	0.00%	0.00%	3,834	3,834	1,617	1,617
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	16,389	16,389	5,824	5,824
@material-ui/core/useMediaQuery	0.00%	0.00%	3,213	3,213	1,311	1,311
@material-ui/lab	0.00%	0.00%	152,627	152,627	46,584	46,584
@material-ui/styles	0.00%	0.00%	51,401	51,401	15,285	15,285
@material-ui/system	0.00%	0.00%	15,666	15,666	4,350	4,350
Button	0.00%	0.00%	79,421	79,421	24,279	24,279
Modal	0.00%	0.00%	15,050	15,050	5,233	5,233
Portal	0.00%	0.00%	3,579	3,579	1,568	1,568
Rating	0.00%	0.00%	70,735	70,735	22,079	22,079
Slider	0.00%	0.00%	75,066	75,066	23,271	23,271
colorManipulator	0.00%	0.00%	3,904	3,904	1,543	1,543
docs.landing	0.00%	0.00%	52,124	52,124	13,837	13,837
docs.main	0.00%	0.00%	591,584	591,584	188,926	188,926
packages/material-ui/build/umd/material-ui.production.min.js	+0.01% 🔺	+0.01% 🔺	300,013	300,030	86,229	86,234

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