Report each server certificate against the config that loaded it (#4664)

`tls_server_cert_serial` reports whichever certificate OpenSSL gave the
lower slot index, not the one in `tls-cert-file`. Slot order is by key
algorithm, so with an RSA and an ECDSA certificate configured, swapping
the two config values produces identical INFO output. Anyone alerting on
`tls_server_cert_expires_in_seconds` silently starts watching a
different file the moment they add an alt certificate. This captures
each certificate's serial and expiry while the context is being built,
which is the last point where the file it came from is known, and
carries those with the context.

This was generated by AI but verified, with love, by a human.

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Signed-off-by: Madelyn Olson <madelyneolson@gmail.com>