Use distinct prompts during Windows WebAuthn registration

[codingllama]

Clearly distinguish between "registered" and "new" devices on Windows "platform" prompts. This is relevant for Windows because it uses the system APIs for both Hello and WebAuthn devices.

I've decided against doing a similar change for Touch ID, as there isn't much to confuse there (lib/auth/touchid doesn't prompt devices other than Touch ID).

Changing the globals is safe because only one WebAuthn prompt happens at a time.

#17563

Changelog: Explicitly mention registered and new device when running tsh mfa add on Windows

[codingllama]

FYI @tobiaszheller.

[codingllama]

A few tests on Windows:

$ .\tsh.exe mfa add --type=WEBAUTHN --name=test
Allow passwordless logins [YES, NO]: no
Using platform authentication for *registered* device, follow the OS dialogs
Using platform authentication for *new* device, follow the OS dialogs
MFA device "test" added.

$ .\tsh.exe webauthnwin diag

WebauthnWin available: true
Compile support: true
DLL API version: 2
Has platform UV: true
Using platform authenticator, follow the OS dialogs
Using platform authenticator, follow the OS dialogs
Register successful: true
Login successful: true

Took me a while to figure out I had to cancel the initial Hello PIN prompt to get it to prompt my Yubikey, but hey it works. (Nothing we can do about that either.)

[tobiaszheller]

lgtm, there is one more issue related to prompt messages on windows: #25051

[public-teleport-github-review-bot]

@codingllama See the table below for backport results.

Branch	Result
branch/v11	Create PR
branch/v12	Create PR
branch/v13	Create PR