Add method option to GenerateSW validation

[chrisdns]

R: @jeffposnick @philipwalton

Fixes #1636

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[chrisdns]

I signed it 👍

[googlebot]

CLAs look good, thanks!

[jeffposnick]

Would you mind adding in validation while we're at it, to match

workbox/packages/workbox-routing/utils/constants.mjs

Lines 35 to 42 in 685fd1b

	export const validMethods = [
	'DELETE',
	'GET',
	'HEAD',
	'PATCH',
	'POST',
	'PUT',
	];

It could be something like

method: joi.string().valid(
  'DELETE',
  'GET',
  '...',
),

[jeffposnick]

Thanks again!