Add data source for pubsub topics

[ScottSuarez]

Upstreaming pr to add data source for pubsub topics: PR 7426

Resolves 6800

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)

`google_pubsub_topic`

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 5 files changed, 154 insertions(+))
Terraform Beta: Diff ( 5 files changed, 154 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=150820"

[danawillow]

Did you use the upstreaming script? I'm surprised not to see the original author listed as a co-committer on this.

Also, take another look at the release notes guide: https://github.com/GoogleCloudPlatform/magic-modules/blob/master/.ci/RELEASE_NOTES_GUIDE.md#examples

[ScottSuarez]

Did you use the upstreaming script? I'm surprised not to see the original author listed as a co-committer on this.

Also, take another look at the release notes guide: https://github.com/GoogleCloudPlatform/magic-modules/blob/master/.ci/RELEASE_NOTES_GUIDE.md#examples

I tried to use the upstreaming script initially but ran into some conflicts even after removing the navigationbar commit. So I switched to manually copy the files for now. I'll try and troubleshoot a little later and possibly add a section to the documentation on resolving these sorts of conflicts if I come to solutions.

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccDataSourceGooglePubsubTopic_basic|TestAccDataSourceGooglePubsubTopic_optionalProject|TestAccProviderMeta_setModuleName|TestAccComputeSnapshot_encryptionCMEK|TestAccMonitoringDashboard_basic|TestAccMonitoringDashboard_gridLayout|TestAccMonitoringDashboard_rowLayout|TestAccMonitoringDashboard_update You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=150832"

[danawillow]

Did you use the upstreaming script? I'm surprised not to see the original author listed as a co-committer on this.
Also, take another look at the release notes guide: https://github.com/GoogleCloudPlatform/magic-modules/blob/master/.ci/RELEASE_NOTES_GUIDE.md#examples

I tried to use the upstreaming script initially but ran into some conflicts even after removing the navigationbar commit. So I switched to manually copy the files for now. I'll try and troubleshoot a little later and possibly add a section to the documentation on resolving these sorts of conflicts if I come to solutions.

Cool, when I run into those issues I often just update the commit to include the other author so they can still get the credit.

[google-cla]

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In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

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[danawillow]

LGTM, also I fixed the release note

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 5 files changed, 154 insertions(+))
Terraform Beta: Diff ( 5 files changed, 154 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=150833"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccProviderMeta_setModuleName|TestAccComputeSnapshot_encryptionCMEK|TestAccMonitoringDashboard_gridLayout|TestAccMonitoringDashboard_basic|TestAccMonitoringDashboard_rowLayout|TestAccMonitoringDashboard_update You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=150836"