Child Workflow creation API should allow user to specify a custom data converter

Is your feature request related to a problem? Please describe.
The user starts a child workflow on a different namespace or task queue, so it will be handled by a different worker.
This different worker may have a completely different configuration of data converters.
Such a situation is pretty legit if we have two different teams developing different modules listening on different task queues.
Right now there is no way for a parent workflow to specify a different data converter for such a child workflow invocation.

Describe the solution you'd like

ChildWorkflowOptions should allow specifying a custom data converter that may be different from the data converter of the current worker.

Describe alternatives you've considered
Currently, users may work around this problem by creating a PayloadConverter that applies only to an input class of the child workflow. This approach is hacky and has limitations.

Additional context
The same applies to ActivityOptions. Activities can't be scheduled on another namespace but could be on a different task queue.

No file or test is named in the issue. Start by locating ChildWorkflowOptions and ActivityOptions, then trace how the worker's data converter is selected for child workflow and activity invocations; done means the relevant options can specify a converter independently and coverage verifies that selection.