How to pass codecs to plugins

Hi,
during my work on #4533 I noticed that we have actually two kind of behaviours for passing codec instances during plugin creation time, basically for a config like:

```
output {
stdout {
codec => plain
}
}
```

you will receive in the `LogStash::Plugin` `initialize` method a hash like `{ "codec" => "plain" }`, but for something like:

```
output {
stdout {
codec => plain { format => "." }
}
}
```

we get the actual instance, see `{"codec"=>".", charset=>"UTF-8">}`

Looks like the actual approach is to pass an instance if the plugin gets parameters or to pass a string if not, leaving the instance creation for later steps.

A good improvement here would be to pass always the same thing, so later steps don't have to create the codec if an string is passed.

Would it be a good option, in your opinion to pass here a hash that include the class + the parameters necessary to build it and let the plugin decide when to create a new instance? create a clone? etc... Other options are also to pass always an instance, but at my understanding passing letting plugin manage their codec instance could be the best approach.

What do you think?