feat: refactor CLI to use direct API calls

[eliasecchig]

• Add get_project_number() function using Resource Manager API
• Replace gcloud projects describe calls with direct API calls in cicd.py
• Add _build_api_headers() helper in register_gemini_enterprise.py
• Include User-Agent and x-goog-api-client headers in Discovery Engine API calls

[gemini-code-assist]

Summary of Changes

Hello @eliasecchig, 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 streamlines the CLI's interaction with Google Cloud by shifting from gcloud CLI commands to direct API calls for retrieving project numbers. It also centralizes and enhances the construction of API request headers for Discovery Engine services, ensuring consistent client identification and reducing boilerplate code.

Highlights

• Direct API calls for project numbers: Replaced gcloud projects describe commands with direct calls to the Resource Manager API for fetching Google Cloud project numbers, reducing reliance on the gcloud CLI.
• Standardized API headers: Introduced a new helper function _build_api_headers to consistently construct headers for Discovery Engine API requests.
• Enhanced API client identification: Incorporated User-Agent and x-goog-api-client headers into Discovery Engine API calls, improving client telemetry and identification.

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[gemini-code-assist]

Code Review

This pull request is a great improvement, refactoring gcloud CLI calls to direct API requests. This enhances performance and reduces external dependencies. The new get_project_number function and _build_api_headers helper are well-implemented. However, the refactoring appears to be incomplete. The get_project_number function within agent_starter_pack/cli/commands/register_gemini_enterprise.py still relies on the gcloud command and should be updated to use the new API-based implementation from agent_starter_pack/cli/utils/gcp.py. This also means its call sites need to be updated to handle exceptions instead of a None return value. I've also added a few specific comments to improve error handling and code structure.