Add DNS ManagedZone managed resource

[danielinclouds]

Description of your changes

Add DNS ManagedZone managed resource.

Fixes #455

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

• Manually tested creating, updating and deleting Managed Zone in GCP.
• Developed unit tests

Notes

This resource is based on ManagedZone but is missing DnssecConfig, ForwardingConfig, NameServerSet, PeeringConfig, ServiceDirectoryConfig, ReverseLookupConfig parameters. The idea behind this implementation is to satisfy the most common use of GCP DNS Managed Zone.