Simplest implementation:

- Put subkeys in an array in your profile, key = "subkey"
- If it is a single string, it is treated as array of length 1
- It is already signed by the user and pulled in by all clients
- Clients can optionally respect it
- Visual indicator on the UI, to show it's a subkey of parent
- [Optionally?] parent can go in the subkey profile
- Other things can be layered on in time, such as revocation, rotation, time stamping, and so on