kpt fn sink should allow merge strategies

[frankfarzan]

Similar to kpt pkg update.

It's not desirable to always overwrite the sink directory.

Note that kpt fn run also implicitly invokes sink.

[monopole]

Do you have a specific example in mind?

[mortent]

@monopole
So an example would be using kpt with Helm. I use a kpt function to hydrate the Helm chart into yaml and I take the result as a Kpt package. This means I can create setters/substitutions and maybe make some manual changes. Eventually there will be a new version of the Helm chart and I would like to update my kpt package. I would like to run the same kpt function again to hydrate the Helm chart, but I also need the merge logic from kpt pkg update to incorporate the changes into my package.
The workaround for this now is that I can make a intermediate git repo where I commit the hydrated Helm chart before using that as the source for updating my kpt package. This seems unnecessarily cumbersome.