Introduce GceInstance that extends cloudprovider.Instance with NumericId

[scotch-bonnet]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Allows access to numeric instance id in GCE Cloud Provider implementation.

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: scotch-bonnet (0673eb0)

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[BigDarkClown]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: BigDarkClown, scotch-bonnet

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