Support multiple shared_key

[hatrena]

Is your feature request related to a problem? Please describe.

I am providing a secure fluentd server as a log aggregator using shared_key. There is also a possibility that a user is able to renew the key. in the meantime, Fluentd is rejecting the log stream due to the wrong shared_key because the user has not yet changed the old key with the new one. so after a minute or so, I update the fluentd to use the new shared_key only.

Describe the solution you'd like

So, It would be great if we are able to configure the <security> snippet to allow multiple shared_key separated by a comma.

Describe alternatives you've considered

An example of supporting multiple shared_key:

<source>
    @type forward
    bind "0.0.0.0"
    port 24224
    <security>
      self_hostname "115134227.local"
      shared_key key_one,key_two
    </security>
</source>

Additional context

No response