Support for mounting verified merged CIMs

[ambarve]

This PR adds two commits:

1. Support for mounting merged verified CIMs
   This commit adds the new CimFS API that mounts the merged verified CIMs. Merge mounting
   verified CIM requires passing the root hash of the merged CIM. All reads happening on this
   mounted verified CIM will then be compared with this root hash for integrity check.
   This commit also adds the tests for the same. (A test that simply mounts verified CIMs was
   removed as that is being tested as a part of other verified CIM tests.) GCS sidecar code is also updated to use the newly added verified CIM mount APIs.

2. Make copies of the VMGS for each confidential pod
   Even though VMGS files aren't written to, we can't share the same file across multiple
   pods because HCS seems to be taking an exclusive lock on the file. To handle this, we make
   copies of the VMGS file (similar to how we make copies of the EFI VHD) per pod. This
   commit also changes the name of the default VMGS file to cwcow.snp.vmgs to better convey
   that by default we run in SNP mode. If a different VMGS file is desired,
   "io.microsoft.virtualmachine.wcow.gueststatefile" annotation can be used to override
   that. This commit also adds the code to handle the UVM console pipe annotation, if provided, for
   windows pods.

[Copilot]

Pull Request Overview

This PR adds support for mounting verified merged CIMs with integrity validation and improves VMGS file handling for confidential pods. The implementation includes new mounting APIs that verify CIM integrity against root hashes, ensuring secure file system operations.

• Implements MountMergedVerifiedBlockCIMs API for mounting merged CIMs with root hash verification
• Updates VMGS file handling to create per-pod copies instead of sharing a single file
• Consolidates UVM console pipe annotation handling to work for both WCOW and LCOW

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pkg/cimfs/mount_cim.go	Adds new API for mounting merged verified CIMs with integrity checking
pkg/cimfs/cim_writer_windows.go	Fixes error reporting and adds path normalization for sealing operations
pkg/cimfs/cim_test.go	Removes redundant test and adds comprehensive test coverage for merged verified CIMs
internal/winapi/zsyscall_windows.go	Adds system call binding for CimMergeMountVerifiedImage API
internal/winapi/cimfs.go	Declares new Windows API function for verified merged CIM mounting
internal/uvm/create_wcow.go	Updates default VMGS filename to indicate SNP mode
internal/oci/uvm.go	Moves console pipe annotation handling to common function for both platforms
internal/gcs-sidecar/handlers.go	Updates to use new verified mounting APIs instead of regular mounting
cmd/containerd-shim-runhcs-v1/pod.go	Implements VMGS file copying and improves error messages

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