Support for wildcard `topic_key`

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

If I have a wildcard `` for `@type kafka2`, it would be great to have a way for `topic_key` to simply mirror the actual topic that is picked up.

### Describe the solution you'd like

```

@type kafka2

...

# write events to this kafka topic
topic_key TODO # < HERE

@type file
path /var/log/td-agent/buffer/td
flush_interval 10s

default_topic TODO

```

### Describe alternatives you've considered

none

### Additional context

none