Add email notifications and audit events for passkey creation & deletion

What does this MR do and why?

Related to #569463 (closed)

This MR adds email notifications and audit events for passkey creation & deletion.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable :passkeys feature flag.

Feature.enable(:passkeys)

2. Open /-/profile/two_factor_auth.

• Add a passkey
• Delete the passkey

3. Open /rails/letter_opener/, you should see messages about the passkey creation & deletion.
4. Open /admin/audit_logs as instance administrator, you should see audit events about the passkey creation & deletion.

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