These changes add support for provider-side previews of create and
update operations, which allows resource providers to supply output
property values for resources that are being created or updated during a
preview.
If a plugin supports provider-side preview, its create/update methods
will be invoked during previews with the `preview` property set to true.
It is the responsibility of the provider to fill in any output
properties that are known before returning. It is a best practice for
providers to only fill in property values that are guaranteed to be
identical if the preview were instead an update (i.e. only those output
properties whose values can be conclusively determined without
actually performing the create/update operation should be populated).
Providers that support previews must accept unknown values in their
create and update methods.
If a plugin does not support provider-side preview, the inputs to a
create or update operation will be propagated to the outputs as they are
today.
Fixes#4992.
249140242e