Expose more metadata during validation

[joeduffy]

We are lacking a number of important pieces of information in policy validation requests (this is already known generally, e.g., https://github.com/pulumi/pulumi-policy/issues/78 and https://github.com/pulumi/pulumi-policy/issues/110).

In attempting to implement an OPA bridge, I've run into further missing pieces, such as the operation being performed (create, update, delete) and the before/after state of the object (during an update). It may be usefull to consult the data structures used by the Kubernetes ValidatingAdmissionWebhook (see here: https://github.com/kubernetes/kubernetes/blob/9905a33fd738a52d3f7ae73cf823d82a05720e33/pkg/apis/admission/types.go#L42).

[ekrengel]

@clstokes What are your thoughts on this? Is it something we should prioritize?