Enable dual stack

[barbacbd]

What type of PR is this?
/kind api-change
/kind feature

What this PR does / why we need it:
Adding the ability to enable dual stacks.

Subnets are one of the resources that control the ability to use a dual stack (IPV4
and IPV6). These are required to be set on resource creation, and they cannot be
altered post creation.

The user has the option to set the stack type for each subnet (IPV4_ONLY, IPV4_IPV6, or
IPV6_ONLY). The default will be IPV4_ONLY.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes # https://issues.redhat.com/browse/CORS-3762

Special notes for your reviewer:

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

 Adding the ability to enable dual stacks.
    
Subnets are one of the resources that control the ability to use a dual stack (IPV4 and IPV6). These are required to be set on resource creation, and they cannot be altered post creation.
    
The user has the option to set the stack type for each subnet (IPV4_ONLY, IPV4_IPV6, or IPV6_ONLY). The default will be IPV4_ONLY.

When creating compute.Subnetwork structs, set the StackType to the same value as the StackType in the SubnetSpecs.

[k8s-ci-robot]

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This pull-request has been approved by: barbacbd
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[k8s-ci-robot]

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