add secret manager cmek

[c2thorn]

Upstreams hashicorp/terraform-provider-google#8976

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).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

secretmanager: added support for setting a CMEK on `google_secret_manager_secret` 

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[c2thorn]

@SKAUL05 mind taking a look at the google CLA?

[SKAUL05]

@googlebot I consent.

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 3 files changed, 142 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 4 files changed, 143 insertions(+), 2 deletions(-))
TF Conversion: Diff ( 1 file changed, 30 insertions(+))

[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=185101"

[modular-magician]

Oops! It looks like you're using an unknown release-note type in your changelog entries:

• REPLACEME

Please only use the types listed in https://github.com/GoogleCloudPlatform/magic-modules/blob/master/.ci/RELEASE_NOTES_GUIDE.md.

[c2thorn]

/gcbrun

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 3 files changed, 142 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 3 files changed, 142 insertions(+), 1 deletion(-))
TF Conversion: Diff ( 1 file changed, 30 insertions(+))

[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=185102"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccComputeBackendService_internalLoadBalancing|TestAccComputeInstance_serviceAccount_updated0to1to0scopes|TestAccSecretManagerSecret_cmek You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=185104"