Introduce Flush for topic producer

[PeterN]

Is your feature request related to a problem? Please describe.
Depending on my business logic I might want to guarantee my data is sent without closing down the topic for future publishing.

Describe the solution you'd like
Having the option to Flush for topic would be beneficial, similar to how there is flush for stream or parts of a stream. With this I wouldn't be forced to close/dispose the topic to guarantee a send.

Describe alternatives you've considered
Other ideas that occurred to me are less mainstream. Flush sounds like the ideal thing considering it is already how it is done elsewhere in the lib.