feat(cdk/drag-drop): Proposal for Keyboard and Screenreader Accessibility

### Feature Description

Hello everyone,

Accessibility is nowadays a very important topic and therefore I think it is essential to get as much work as possible done directly in the framework. Therefore is my proposal to make the Angular CDK Drag and Drop accessible to everyone! My ideas so far

- Reusing as much code as possible (maybe renaming the functions if selected)
- Adding aria roles if necessary
- tabindex to all grabbable items
- Space to activate moving by keyboard (then activly moving by arrow keys with small movement). Further ideas/improvement: movement to fix containers or movement by bigger (user defined) steps!
- Escape to quit movement
- Implementation according to https://www.w3.org/wiki/PF/ARIA/BestPractices/DragDrop

First Code proposal (Please don't look to specific, is my first proposal, Improvements needed):
https://github.com/angular/components/commit/1c50475095a7ecf3c1f142c820d3f5a71f0070b8

Is is meaningful to do more work on it? Please be so kind and share your meanings with me!

Best regards,

Martin

### Use Case

Drag and Drop Accessibility!

Start with the Angular CDK Drag and Drop entry point and review the linked code proposal alongside the W3C ARIA DragDrop best practices. Clarify the accepted keyboard, screenreader, focus, and movement behavior before identifying the implementation and test scope; done means an agreed accessibility design and verified behavior for the proposed interactions.

Written by the indexing model from the issue text.