Observe Only - POC

[turkenh]

Description of your changes

This is just a POC PR for the required changes in upjet for the Observe Only proposal.

Changes in this PR will probably be split into multiple PRs:

• Include all state under status.atProvider, i.e. status.atProvider is a superset of spec.atProvider. See the discussion leading to this in addition to what is already proposed in the design.
• Define CEL rules to mark fields as required depending on the management policy.
• Usage of Terraform import when management policy is Observe Only.

I have:

• Read and followed Crossplane's [contribution process].
• Run make reviewable to ensure this PR is ready for review.
• Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

Together with provider-gcp, by generating new schemas and observing resources.

[turkenh]

Closing this POC PR in favor of the following PRs:

• Include full state under status.atProvider #164
• Generate CEL validation rules not to enforce required fields when ObserveOnly #166
• Observe with Terraform Import when Observe Only #168