For functions, allow users to configure whether the output should inherit the key of the input

[jerrypeng]

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

Currently, for Java functions, if the input message has a key associated with it, it is automatically inherit onto by the output message.

This might not be the desirable behavior for all scenarios.

In Python functions, the keys of input messages are not inherited by output messages.

Describe the solution you'd like

Allow users to configure that behavior. Make behavior of Python and Java functions consistent

[sijie]

This issue has been implemented with the flag forward message properties.