Add WebAuthn/Passkey selector support in config flows for integrations

[holyhope]

Describe your core improvement

Summary

Add a new WebAuthnSelector to the config flow system that allows integrations to request WebAuthn/Passkey authentication from users during setup or reauthentication.

Proposed Solution

Add a new selector type that works within async_show_form:
Backend (homeassistant/helpers/selector.py):

Alternative Considered

Using async_external_step to redirect to a custom HTML page that handles WebAuthn. This works but:

1. Creates a fragmented user experience (opens new window/tab).
2. Requires each integration to implement its own WebAuthn page.
3. Doesn't follow the standard config flow pattern.

Related Discussions

Community: WebAuthn authentication (passkeys, security keys)
Architecture: Open letter for improving Home Assistant's Authentication system #832

Current limitations

Some third-party services now require or support FIDO2/WebAuthn security keys for authentication.

For example:

• Apple iCloud supports Security Keys for Apple Account as of iOS 16.3/macOS 13.2
• The upstream pyicloud library already exposes WebAuthn challenge data (security_key_names, fsaChallenge) and can verify assertions.

Currently, the icloud integration cannot support users with security keys enabled on their Apple accounts because:

• The config flow system has no mechanism to trigger browser WebAuthn APIs (navigator.credentials.get())
• There's no WebAuthnSelector that can pass challenge data to the frontend and receive signed assertions back.

  @SELECTORS.register("webauthn")

class WebAuthnSelector(Selector[WebAuthnSelectorConfig]):
"""Selector for WebAuthn/Passkey authentication."""

selector_type = "webauthn"
# Config includes: challenge, rpId, allowCredentials, userVerification, timeout

### Technical benefits

This would introduce the logic of web authentication to the front end, and leverage future features:
- iCloud integration: Support Apple accounts with security keys
- Future integrations: Any service adopting WebAuthn/Passkeys for 2FA
- Potential HA login: Could eventually be used for Home Assistant's own authentication (as discussed in [the community feature request](https://community.home-assistant.io/t/webauthn-authentication-passkeys-security-keys/439166)).

### Additional context

I'm working on adding security key support to the `icloud` integration and discovered that `pyicloud:2.2.0` already exposes the necessary `WebAuthn` challenge data internally. The backend pieces are available - we just need frontend support to complete the flow.
I'd be happy to contribute to this feature if there's interest from maintainers.