WIP: fine-grained address parts requests

[marcoscaceres]

closes #842

This is a rough proposal - just putting in place the IDL

The following tasks have been completed:

• [x] Confirmed there are no ReSpec errors/warnings.
• [ ] Modified Web platform tests (link)
• [ ] Modified MDN Docs (link)
• [ ] Has undergone security/privacy review (link)

Implementation commitment:

• [ ] Safari (link to issue)
• [ ] Chrome (link to issue)
• [ ] Firefox (link to issue)
• [ ] Edge (public signal)

Optional, impact on Payment Handler spec?

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[zouhir]

This is great - very excited for this idea.

[marcoscaceres]

This is now #873