Support multiple shared_key

### 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 `` snippet to allow multiple `shared_key` separated by a comma.

### Describe alternatives you've considered

An example of supporting multiple `shared_key`:
```

@type forward
bind "0.0.0.0"
port 24224

self_hostname "115134227.local"
shared_key key_one,key_two

```

### Additional context

_No response_

Start at the forward input's configuration and the existing shared_key validation, then inspect related tests. Clarify how key rotation should accept both old and new keys and how comma-separated values are parsed. Done means the configuration accepts multiple keys, authentication behavior is covered by tests, and existing single-key use still works.

Written by the indexing model from the issue text.