feat: Support direct image access

[NautiluX]

When bootstrapping test infra, allow pulling from image registry when PAT is set.

Clickup: https://app.clickup.com/t/869c079a0

[Copilot]

Pull request overview

This PR adds support for direct image access from GitHub Container Registry when a Personal Access Token (PAT) is provided, enabling more efficient bootstrapping of test infrastructure by pulling container images directly instead of bundling them in installation packages.

Changes:

• Added GitHub registry type support with PAT-based authentication
• Modified VM disk sizes to be smaller (50GB vs 200GB) when using direct GitHub access
• Updated installer to use lite packages when GitHub PAT is configured

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
internal/bootstrap/gcp/gcp.go	Added GitHub registry type constant, PAT/user fields, GitHub access configuration method, and conditional logic for package selection and disk sizing
internal/bootstrap/gcp/gcp_test.go	Refactored test setup to share mock objects across test cases and added tests for GitHub access configuration
cli/cmd/bootstrap_gcp.go	Added CLI flags for GitHub PAT and registry user, with validation logic
docs/oms-cli_beta_bootstrap-gcp.md	Updated documentation to include new GitHub PAT and registry user flags

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