Implement KeyboardFocus resource and focus keyboard events

[viridia]

What problem does this solve or what need does it fill?

See discussion: #15374

What solution would you like?

Implementation steps:

• [ ] Define KeyboardFocus resource.
• [ ] Write synchronization code between KeyboardFocus and bevy_a11y::Focus.
• [ ] Define FocusKeyboardEvent and HoverKeyboardEvent types.
• [ ] Write systems to read from input queue and dispatch the events.
• [ ] Write plugin which adds these systems and resources.

@alice-i-cecile @UkoeHB

[viridia]

Prototype is in PR #15611