[WIP] Extend the API to expose the options required to provision an instance with GPU attached

[aniruddha2000]

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

What this PR does / why we need it:
Extend the API changes required for GCPMachineSpec to expose the options needed to provision a GCP computer instance with GPU(s) attached.

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 #289

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:

Extend the API to expose the options required to provision an instance with GPU attached

[k8s-ci-robot]

Hi @aniruddha2000. Thanks for your PR.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: aniruddha2000
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[aniruddha2000]

/cc @richardcase @cpanato @dims