Offer legacy contacts for deceased / inactive users

### What

We should offer a way for users to add "legacy contacts", or someone who gets access to a WordPress.com account when someone has become deceased or inactive after a certain amount of time. Both [Apple](https://digital-legacy.apple.com/) and [Google](https://support.google.com/accounts/answer/3036546) have similar systems in place.

### Why

There may be cases where users want to designate someone to have access to their account or site after they become inactive or deceased.

Request on 5271514-zen

### How

I would refer to Apple/Google to see how we can potentially implement a system.

No files, tests, or entry points are identified in the issue. Start by reviewing the Apple and Google legacy-contact references, then define the account-access rules, inactivity or death criteria, and completion requirements before locating the relevant implementation areas.

Written by the indexing model from the issue text.