Correct data type for Notebook Instance Owners field, add some enums

[gogasca]

Add Initialization state
Getting: Error:

Error reading Instance: instance_owners: '' expected type 'string', got unconvertible type '[]interface {}'

Currently Instance owners is type Array with string items
https://cloud.google.com/ai-platform/notebooks/docs/reference/rest/v1beta1/projects.locations.instances#Instance

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• [X ] Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

notebooks: fixed broken `google_notebooks_instance.instance_owners` field by making it a list instead of a string

notebooks: added `PD_BALANCED` as a possible disk type for `google_notebooks_instance`

[modular-magician]

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[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform Beta: Diff ( 1 file changed, 4 insertions(+), 1 deletion(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=142511"

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform Beta: Diff ( 1 file changed, 4 insertions(+), 1 deletion(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=142527"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccActiveDirectoryDomainTrust_activeDirectoryDomainTrustBasicExample|TestAccActiveDirectoryDomain_activeDirectoryDomainBasicExample|TestAccComposerEnvironment_update You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=142528"

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform Beta: Diff ( 2 files changed, 12 insertions(+), 9 deletions(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=142784"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccComputeDisk_VSSWindows You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=142798"

[danawillow]

LGTM, thanks! I updated the changelog note a bit, you can take a look if you're curious.

[gogasca]

Thanks Dana!