Update secret_values to support dynamic elements within parent

What does this MR do?

EE MR, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4222

Part of the dynamic CI variable list, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4110

• Update secret_values to support dynamic elements within parent
  • In the context of the dynamic variable list, rows are dynamic
• Add valueSelector/placeholderSelector options
  • In the context of the dynamic variable list, we use this to avoid selecting the :last-child row

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
  • Not a user-facing change
• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

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