Add OIDC auth to Mail API

[hkeeler]

Similar to our other FastAPI services, this is going to require OIDC-based authorization in order to send email through the
API. And once we have that working, we can also replace the HTTP headers (X-Mail-From-Address and X-Mail-From-Name at time of writing) with content from the OIDC access token. This will prevent users from sending email on another user's behalf.