expose signer key id argument in zts-svccert utility

[havetisyan]

Description

Contribution Checklist:

• The pull request does not introduce any breaking changes
• I have read the contribution guidelines.
• Create an issue and link to the pull request.

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[gemini-code-assist]

Summary of Changes

Hello @havetisyan, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the zts-svccert utility by providing a new command-line option to specify the signer key ID. This change allows for greater control and flexibility during certificate issuance, enabling users to select a particular signing key for their certificates, which is particularly useful in environments managing multiple certificate signing keys.

Highlights

• New Command-Line Argument: Introduced a new command-line argument, -signer-key-id, to the zts-svccert utility.
• Signer Key ID Specification: The new argument allows users to explicitly specify the key ID of the certificate signer when requesting a certificate.
• Updated Usage Information: The utility's usage message has been updated to reflect the availability of the new -signer-key-id flag.

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[gemini-code-assist]

Code Review

This pull request exposes a new -signer-key-id command-line argument in the zts-svccert utility. The changes correctly add the new flag and use its value to populate the X509CertSignerKeyId field for InstanceRegisterInformation requests.

My review includes a couple of points:

1. A minor correction to the usage string to fix some missing brackets.
2. A suggestion to extend the support for the new -signer-key-id flag to InstanceRefreshRequest as well, to provide consistent functionality for both certificate request paths in the utility.

Overall, the changes are in the right direction.