Add missing usedBy field to Certificate type in certificate_manager_v1

[mervin008]

The REST API Certificate resource exposes a usedBy[] field (field number 10) of type UsedBy, but the Python client's Certificate type does not include it.

Changes

• Added Certificate.UsedBy inner message class with a name string field (field number 1)
• Added Certificate.used_by repeated field (field number 10)
• Updated Certificate class docstring to document the new field
• Fixed ip_configs field formatting in CertificateMap.GclbTarget (minor syntax fix)

Checklist

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #17134 🦕

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[gemini-code-assist]

Code Review

This pull request updates the Certificate message in the Certificate Manager library by adding a used_by field, which tracks resources utilizing the certificate. The feedback suggests reordering the newly added field to maintain numerical consistency with existing fields and removing excessive blank lines between class definitions to adhere to PEP 8 standards.