Add AttachDisk/DetachDisk RPCs to VirtualMachineAgent

[raghavendra-nataraj]

Summary

Adds two dedicated cloud-agent gRPC RPCs to the VirtualMachineAgent service for attaching and detaching virtual hard disks on an existing virtual machine:

rpc AttachDisk(VirtualMachineDiskRequest) returns (VirtualMachineDiskResponse) {}
rpc DetachDisk(VirtualMachineDiskRequest) returns (VirtualMachineDiskResponse) {}

New messages:

message VirtualMachineDiskRequest {
    string groupName = 1;
    string vmName = 2;
    string locationName = 3;
    repeated Disk disks = 4;
}

message VirtualMachineDiskResponse {
    VirtualMachine virtualMachine = 1;
    google.protobuf.BoolValue Result = 2;
    string  Error = 3;
}

Why

Today, CSI attaches/detaches disks by calling VirtualMachineAgent.Invoke with Operation_POST (CreateOrUpdate). That path can recreate a VM after it has been deleted (a create/delete race), which is unintended, and it validates/reconciles the VM's entire disk set.

VirtualMachineDiskRequest carries the target VM identity plus only the disks to act on (not the VM's full storage configuration). This lets the cloud agent:

• verify the VM exists and refuse to (re)create it, and
• operate on exactly the passed disks.

Scope

This PR is the proto + generated code foundation only. The cloud-agent handler/provider/client implementation (VM-existence guard, per-disk attach/detach) will land in a follow-up wssdcloudagent PR once this change is released and the module version is bumped there.

RBAC: the follow-up handler will reuse ProviderAccessOperation_Unspecified (as RunCommand does), so no moc_common_common.proto enum change is needed here.

Codegen

moc_cloudagent_virtualmachine.pb.go regenerated with protoc 3.11.4 + protoc-gen-go v1.3.2 (matching the repo toolchain; a no-op baseline regen was byte-identical to the previous generated file before this change). go build ./... and go vet ./rpc/cloudagent/compute/ pass.

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