refactor(@angular/cli): migrate ng add to new package manager abstraction

[clydin]

This commit fully refactors the ng add command to use the new, centralized package manager abstraction.

All package installation and metadata fetching logic now goes through the new API. This includes:

• Using createPackageManager to detect the project's package manager.
• Replacing fetchPackageMetadata with packageManager.getRegistryMetadata.
• Replacing fetchPackageManifest with packageManager.getManifest.
• Replacing the install and installTemp methods with the packageManager.add and packageManager.acquireTempPackage methods.