security-command-center: no code samples for managing security marks on assets

## In which file did you encounter the issue?

https://github.com/GoogleCloudPlatform/java-docs-samples/tree/dd24a493/security-command-center/snippets/src/main/java/vtwo/marks

## Did you change the file? If so, how?

no

## Describe the issue

There are no code samples that show how to use Security Command Center v2 to manage security marks for assets. We only have code samples for security marks on findings.

We need v2 equivalents of several v1 snippets from this file: https://github.com/googleapis/google-cloud-java/blob/cff95fd0631777a9a8f077848c39d458c7e5b339/google-cloud-examples/src/main/java/com/google/cloud/examples/securitycenter/snippets/SecurityMarkSnippets.java

Specifically, we need v2 equivalents of the samples with the following region tags:

+ `securitycenter_add_security_marks`
+ `securitycenter_delete_security_marks`
+ `securitycenter_add_delete_security_marks`

cc: @owenhuyn