Discoverable passkey upgrade

Use the same UVPA with discoverable credential support (e.g. Touch ID or Windows Hello) for all the following steps:

Step #	Action	Expected	Actual	Matches Expected	Details
1.
Simulate an old security key registration
2.
3.
Attempt to register as discoverable passkey
4.
Upgrade security key registration to discoverable passkey
5.
6.
Verify discoverable passkey registration
7.
8.

This journey simulates the following situation:

An RP has historically supported "security keys", and a user has registered a platform authenticator as security key (steps 2-3).
The RP has added discoverable passkey functionality.
The user is now registering the same platform authenticator as a discoverable passkey, and the RP needs to make sure that this doesn't silently break the security key registration (steps 4-6).
- If the RP did not take care, most major browsers would overwrite the security key registration with the passkey registration, but there would be no way to tell. The user could plausibly think "I can remove this passkey registration, it's still registered as a security key", only to find that the security key registration is no longer possible to use at all.

Informally, the RP is "upgrading" is platform authenticator from a security key registration (UVPA/RK status potentially unknown) to a discoverable passkey registration in step 6.

Note that this particular flow still has a flaw: there is no guarantee to the RP that that the device identified in step 5 is the same one registered in step 6. This is not an issue in practice, so long as device supports at most platform authenticator. However, it would be nice to address this issue while also improving the UX of this flow, by consolidating steps 4-6.

Step 2 uses:
"authenticatorSelection": { "userVerification": "discouraged" }

Steps 4 and 6 use:
"authenticatorSelection": { "authenticatorAttachment": "platform", "userVerification": "required" "requireResidentKey": true }

Step 7 uses:
"allowCredentials": []

Step 8 uses:
"allowCredentials": [/* registration from step 6 */]

Currently stored registrations: