chore: add native image sample for google-cloud-secretmanager

[mpeddada1]

TODO: Add test job to handle verification of native image sample.

[mpeddada1]

The generation diff / root-pom check failure is expected as this sample module hasn't been included in https://github.com/googleapis/sdk-platform-java/blob/main/library_generation/templates/root-pom.xml.j2 yet

(thanks for the context @JoeWang1127)

[mpeddada1]

The presubmit is detecting changes in the sample:

[Edit] This is expected behavior.

[suztomo]

@JoeWang1127 How does the hermetic build script handles the sample directory?

[JoeWang1127]

@JoeWang1127 How does the hermetic build script handles the sample directory?

Hermetic build won't affect java-samples because it's not listed in generation_config.yaml

[burkedavison]

This LGTM, but I'd like to have @suztomo 's approval before merge.

[mpeddada1]

Thank you! Confirmed by running the stage job with this PR's commit that we aren't staging any of the sample artifacts.