Ability to add non-breadcrumb links to a header

[ben-polinsky]

Feature

The iTwinUI Header allows consumers to add links via a breadcrumbs prop. This displays the menu items with carets pointing to the next item to the right, making them feel logically connected. While this suits iTwin applications which display iTwins containing iModels containing versions, it limits apps displaying more traditional headers.

Examples

The current Connect Header has header items which do not feel connected in a bread-crumby fashion.

[mayank99]

This will likely require breaking changes if we update Header. Or we could create a new version from scratch (HeaderNew) after releasing v3, and deprecate the old Header.

/cc @iTwin/itwinui-1