Split seekable files from non-seekable files

[djeebus]

This allows us to cache, compress, etc.

• Add a new OpenSeekableObject method to the storage provider that is used to read seekable files
• Remove the ReadAt method from the ObjectProvider interface (only used by SeekableObjectProvider).
• Remove the Delete method from the ObjectProvider interface (never used).
• Add a SeekableObjectProvider interface.
• Add new ObjectType and SeekableObjectType types that defines which file type is being passed in, in order to help the compiler prevent mistakes

---

Note

Introduces separate seekable/non-seekable storage interfaces with typed OpenObject/OpenSeekableObject, updates providers, caching, orchestrator code, mocks, and tests accordingly.

• Storage API:
  • Split interfaces into ObjectProvider (non-seekable) and SeekableObjectProvider (seekable); removed ReadAt/Delete from ObjectProvider, added Exists.
  • Added ObjectType and SeekableObjectType enums; updated StorageProvider with OpenObject(ctx, path, ObjectType) and OpenSeekableObject(ctx, path, SeekableObjectType).
  • Refactored caching: new CachedObjectProvider and CachedSeekableObjectProvider; moved chunked read/size logic to seekable cache; reorganized cache helpers.
• Providers (GCP/AWS/FS):
  • Implement new interfaces and methods; added Exists; adjusted OpenObject/OpenSeekableObject signatures.
• Orchestrator:
  • Updated commands and template code to pass object kinds and use OpenSeekableObject for memfile/rootfs and OpenObject for headers/metadata/snapfile.
  • storage_diff: constructor now returns (*StorageDiff, error); added diff→object type mapping; use OpenSeekableObject.
  • Template build/upload paths now typed; server upload checks Exists instead of Size.
• Tests & Mocks:
  • Replaced old mocks with mockobjectprovider and mockseekableobjectprovider; updated unit tests for new interfaces and behaviors.
• Config:
  • .mockery.yaml updated to generate mocks for new interfaces.

^{Written by Cursor Bugbot for commit 77c3db5. This will update automatically on new commits. Configure here.}

[dobrac]

All good. Small nits there, so approving!

Just wondering, wouldn't it be better to do instead of:
obj, err := s.OpenObject(ctx, files.StorageMetadataPath(), storage.MetadataObjectType)

something like this?
obj, err := s.OpenObject(ctx, storage.MetadataObjectType{ Path: files.StorageMetadataPath()})
(we could then return the storage.MetadataObjectType directly instead of the string path)