[WIP] Add CAPI types for GCPCluster support

[hasheddan]

Description of your changes

Adds the required API types to support GCPCluster in cluster-api-provider-gcp.

Signed-off-by: hasheddan [email protected]

Fixes #303

I have:

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

How has this code been tested

Very much still a WIP.

TODO:

• [ ] Make optional fields pointer types
• [ ] Implement update / late init for all API types
• [ ] Implement clients and controllers for BackendService, TargetTcpProxy, ForwardingRule, InstanceGroup, Firewall
• [ ] Add unit tests for all clients and controllers