[poc] DO NOT MERGE: shared account ID is off curve

[s8sato]

Description

• Proof of concept for the new account, with non-essential parts omitted
  • There is room for optimization in actual implementation
• This is in the form of a draft PR just for review and not expected to be merged

Linked issue

• The concept demonstrated here is Add primary key to account #2085 (comment)
• Intended to be one of the foundations for Key-centric accounts  #2083 in general and Reimplement multi-signature #4373 in particular

Benefits

Checklist

• I've read CONTRIBUTING.md
• I've used the standard signed-off commit format (or will squash just before merging)
• All applicable CI checks pass (or I promised to make them pass later)
• (optional) I've written unit tests for the code changes
• I replied to all comments after code review, marking all implemented changes with thumbs up

[s8sato]

• Closed as this was for Reimplement multi-signature #4373, which has been resolved