[Breadcrumbs] Can't expand collapsed Breadcrumbs via keyboard

[malouro]

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

A collapsed set of Breadcrumbs (ie: a maxItems prop is set, and the number of inner items exceeds this value) cannot be expanded via keyboard.

Expected Behavior 🤔

Should be able to focus onto the expand button and hit enter/space to expand the set of collapsed Breadcrumbs.

Steps to Reproduce 🕹

Steps:

1. https://codesandbox.io/s/sxhju
2. Tab to the "Home" Breadcrumb. (Focus should be on the "Home" Breadcrumb now)
3. Hit tab again
4. Focus won't go to the ... button to be able to expand the Breadcrumbs via keyboard

Context 🔦

Users are unable to use the collapse/expand functionality of the Breadcrumbs component via keyboard - which would also be an issue for assistive technology users.

Your Environment 🌎

Tech	Version
Material-UI	latest
React	latest
Browser	Chrome
TypeScript
etc.

[oliviertassinari]

I believe that we also miss a descriptive label, something we could have built-in and localize.

[captain-yossarian]

@oliviertassinari @malouro Can I pick it up ?

[malouro]

@captain-yossarian that's cool with me, sure!