Add a link to OpenID Issuer URL in WebFinger response (#26000)

Add a link to OpenID Issuer URL in WebFinger response (#26000)

This change adds an entry to the `links` field of the
`/.well-known/webfinger` response. The new entry points to the `appURL`
as an OpenID issuer. This is consistent with the output of
`/.well-known/openid-configuration`, which also uses the value of
`appURL`.

This change is required for Tailscale SSO to work, as it uses the OpenID
Issuer URL when querying the Tailscale user email during signup/login
([docs](https://tailscale.com/kb/1240/sso-custom-oidc/#webfinger-setup)).