Add support for HTML element option for Cover block.

[EldarAgalarov]

Group block have HTML ELEMENT option for html tags picking different than div.
Cover block doesn't have such option. Both blocks are container blocks so Cover block should have this option too.

[t-hamano]

I looked up the blocks where the HTML element option is supported. In my opinion, it would be better if the cover block supported 7 elements as well as the group block.

	div	header	main	section	article	aside	footer
core/group	✅(default)	✅	✅	✅	✅	✅	✅
core/template-part	✅	✅	✅	✅	✅	✅	✅
core/comments	✅(default)			✅		✅
core/query	✅(default)		✅	✅		✅

[carolinan]

I'd prefer to exclude <main>, to avoid it being used by mistake?

[t-hamano]

I'd prefer to exclude

, to avoid it being used by mistake?

I think there are users who would like to include a pattern image in the background of the main area. However, it is difficult to say whether this is a common use case.