Add Support for Observe Only Resources

[turkenh]

Description of your changes

This PR adds a flag for Management Policies (a.k.a.) Observe Only resources.

Please note, this PR does not cover representing the full state at status.atProvider which is being tracked with a separate issue: https://github.com/crossplane-contrib/provider-gcp/issues/504

I have:

• [x] Read and followed Crossplane's contribution process.
• [x] Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

1. Run provider with --enable-management-policies
2. Create a resource with spec.managementPolicy: ObserveOnly
3. Verify that it becomes ready & synced after observing.