Fix error in ProvisionerProvider.diff() when inputs are pulumi.output.Unknown

[jamesfmilne]

This patch adds a helper function to handle pulumi.output.Unknown objects when performing a diff on a ProvisionerProvider-based resource.

This in turn fixes issues with pulumi preview and pulumi up when using these providers in a stack when any of the inputs changes.

[github-actions[bot]]

PR is now waiting for a maintainer to run the acceptance tests.

Note for the maintainer: To run the acceptance tests, please comment /run-example-tests on the PR