If a stack output includes a `Resource`, we will as of a recent change
always show the output diff, but this diff will potentially include
unknowns, leading to spurious output like:
```
+ namePrefix : output<string>
```
These changes supress these diffs by adding a special key to the POJO
we generate for resources *during preview only* that indicates that the
POJO represents a Pulumi resource, then stripping all adds of unknown
values from diffs for objects marked with that key.
Fixes#3314.
c383810bf8