feat: VM Indentity samples added.

[m-strzelczyk]

Description

Fixes b/187880839

I made the tests to execute only when running on GCE VM instance (which I assume is happening for the automated tests). I think they are more valuable this way than if I'd use mock to provide responses from the metadata server. Then they wouldn't be able to fail at all IMO.

Checklist

• I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• README is updated to include all relevant information
• Tests pass: nox -s py-3.6 (see Test Environment Setup)
• Lint pass: nox -s lint (see Test Environment Setup)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• Please merge this PR for me once it is approved.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample

[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• compute/metadata/vm_identity.py:16, tag compute_vm_identity_verify_token
• compute/metadata/vm_identity.py:24, tag compute_vm_identity_acquire_token
• compute/metadata/vm_identity.py:49, tag compute_vm_identity_verify_token

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[leahecole]

Hey! I'm so sorry for the delay on looking at this. Can you have a reviewer familiar with Compute products on your team take a look to make sure this is good from the product side? I'll take a look at the tests.

[leahecole]

@engelke I'd actually prefer if you could be the sample owner that takes a look at this just given that you have more experience in this area than I do 😄

[engelke]

LGTM

[m-strzelczyk]

@busunkim96 Could you have a look at this? :)