ci: add shared security scanning workflow

[opentelemetrybot]

Tracked in open-telemetry/sig-security#293.

Adds zizmor scanning through the OpenTelemetry shared workflow.

The workflow:

• runs the regular persona on pull requests, pushes to configured push branches, and a randomized weekly schedule;
• uploads results to GitHub code scanning;
• uses the shared workflow pinned to an immutable commit.

Zizmor remediation

This change also resolves findings from these zizmor rules:

• artipacked
• github-app
• github-env
• template-injection

After merge

1. Confirm the Zizmor workflow succeeds on the merged default-branch commit and its code-scanning analysis has no findings.
2. Mark the corresponding admin PR ready for review and merge it to require zizmor code scanning.
3. On the next repository pull request, confirm GitHub reports a required zizmor code-scanning result.

If enforcement unexpectedly blocks a valid change, restore require_zizmor = false in open-telemetry/admin while leaving scanning enabled.

[github-advanced-security]

You are seeing this message because GitHub Code Scanning has recently been set up for this repository, or this pull request contains the workflow file for the Code Scanning tool.

What Enabling Code Scanning Means:

• The 'Security' tab will display more code scanning analysis results (e.g., for the default branch).
• Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results.
• You will be able to see the analysis results for the pull request's branch on this overview once the scans have completed and the checks have passed.

For more information about GitHub Code Scanning, check out the documentation.