Rust-SDK: Make PendingChildWorkflow and StartedChildWorkflow public

[c-thiel]

What was changed

Make PendingChildWorkflow and StartedChildWorkflow publicly accessible.

Why?

PendingChildWorkflow and StartedChildWorkflow are unreachable pub items.
They provide additional value though in Workflows to spawn / signal / cancel child workflows and should thus be externally accessible.

I also changed the PendingChildWorkflow common visibility field to pub(crate), as the corresponding Datatype is currently also not accessible from the outside. It should be consistent for PendingChildWorkflow and StartedChildWorkflow for external users.

Checklist

1. Closes
   None yet. Do you need one?

2. How was this tested:

Not needed, just pub changes.

4. Any docs updates needed?

No

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