feat: Add Harness GitOps entity support

[dewan-ahmed]

The CLI currently has no support for Harness GitOps. Users managing
GitOps Agents, Clusters, Repositories, Applications, and ApplicationSets
must use the UI or REST API directly.

Adding GitOps support to the CLI would enable users to manage the full
GitOps lifecycle from the terminal and unblock automation/agent use cases.

Suggestion:

Noun	list	get	create	update	delete	execute
gitops_agent	✓	✓	✓	✓	✓	✓ (manifest)
gitops_cluster	✓	✓	✓	✓	✓
gitops_repository	✓	✓	✓	✓	✓
gitops_application	✓	✓	✓	✓	✓	✓ (sync, refresh)
gitops_application_set	✓	✓	✓	✓	✓	✓ (sync)

The gitops_ prefix avoids collision with the existing repository (Code) noun.