feat: add asset management package for AM2.0 support

[naman-contentstack]

Asset Management 2.0 – Export Support

Summary

• Add a new @contentstack/cli-asset-management package that handles exporting space-based asset data when AM 2.0 is enabled for a stack
• The assets export module now checks for linkedWorkspaces (fetched from CMA branch settings am_v2.linked_workspaces during stack export) to decide between the legacy asset flow and the new spaces-based flow
• When AM 2.0 is active, exports org-level fields and asset types (shared), then per-space metadata, folder structure, asset metadata, and downloaded asset files into a spaces/ directory structure
• Legacy export behavior is fully preserved when no linked workspaces are present

What changed

New package: @contentstack/cli-asset-management

• ExportSpaces orchestrator with per-space export classes (ExportWorkspace, ExportAssets, ExportFields, ExportAssetTypes)
• AssetManagementAdapter HTTP client for the AM API (/api/spaces, /api/fields, /api/asset_types)
• Base class with progress tracking and chunked JSON write support

Modified: contentstack-export

• assets.ts: Branches on linkedWorkspaces.length > 0 to delegate to ExportSpaces
• stack.ts: Fetches linked workspaces from branch settings and sets them on exportConfig
• New get-linked-workspaces.ts utility
• Added linkedWorkspaces to ExportConfig and assetManagementUrl to Region type
• Updated progress strategy registry for AM 2.0 process names

Export structure (AM 2.0)

{branch}/spaces/
  ├── fields/fields.json            (shared, org-level)
  ├── asset_types/asset-types.json  (shared, org-level)
  └── {space_uid}/
      ├── metadata.json
      └── assets/
          ├── folders.json
          ├── assets.json
          └── files/{asset_uid}/{filename}