[accordion] add support for meta data in header

[jshenkman]

Design is here

I used a badge for the meta-data

[yinonov]

A long awaited request was previously raised to provide support for badge (or any other meta data text content) within the title button area.

going through some leading libraries, there's not much evidence of such implementations. the only few have showed a strict version of meta where elements are defined and handled by the library maintainers.

BBC accordion, for example, feature meta text to reflect items count

meaning only text content is allowed as part of the title (label).

notice that an implementations such as the following

will cause the accessibility tree to output unclear (but tolerable?) button label - Layout 5

as some designs contain other types of elements in the meta space (other than badges, or different types of badges, outlined / filled), it will require a slot to be exposed.
slots are vulnerable, therefore discouraged where "abuse" or wrong content can produce violations.

after considering all meta forms, and accepting the fact that label will concatenate with the meta text (in the accessibility tree), we should recommend string input only in the form of property, where its UI representation will be specified by the component and not by author.

[yinonov]

relates to #1218

[rachelbt]

design here:
https://tmp.dev.vivid.vonage.com/1225/index.html?path=/story/introduction-meet-vivid--meet-vivid