Make netlogfmt useful regardless of the exact schema of the input.
If a JSON object looks like a network log message,
then unmarshal it as one and then print it.
This allows netlogfmt to support both a stream of JSON objects
directly serialized from netlogtype.Message, or the schema
returned by the /api/v2/tailnet/{{tailnet}}/network-logs API endpoint.
Signed-off-by: Joe Tsai <joetsai@digital-static.net>