Replace AttachDisk/DetachDisk with a single UpdateDisks RPC

[raghavendra-nataraj]

What

Replace the two dedicated cloud-agent RPCs AttachDisk and DetachDisk (added in #441) with a single UpdateDisks RPC on the VirtualMachineAgent service. The direction is selected by a new VirtualMachineDiskOperation enum (ATTACH / DETACH) carried on VirtualMachineDiskRequest.

enum VirtualMachineDiskOperation {
  VIRTUAL_MACHINE_DISK_OPERATION_UNSPECIFIED = 0;
  VIRTUAL_MACHINE_DISK_OPERATION_ATTACH      = 1;
  VIRTUAL_MACHINE_DISK_OPERATION_DETACH      = 2;
}

message VirtualMachineDiskRequest {
  string groupName    = 1;
  string vmName       = 2;
  string locationName = 3;
  repeated Disk disks = 4;
  VirtualMachineDiskOperation operation = 5;
}

rpc UpdateDisks(VirtualMachineDiskRequest) returns (VirtualMachineDiskResponse);

Why

#441 introduced AttachDisk/DetachDisk as two RPCs, but no consumer implements or calls them yet (the wssdcloudagent handler is a follow-up), so this is a safe time to consolidate the surface:

• Single-direction-per-call matches the actual caller. The azhcidisk CSI driver attaches/detaches exactly one disk per ControllerPublishVolume / ControllerUnpublishVolume (each maps to a single DiskAttach / DiskDetach). There is no caller that needs to attach and detach in one atomic request, so a two-list "attach these + detach those" API would be unused complexity. One RPC + an operation selector covers every real caller with a smaller surface.
• Fail-safe default. operation defaults to UNSPECIFIED (proto3 zero value). The handler rejects UNSPECIFIED, so a caller that forgets to set the direction gets an error instead of a silent attach/detach.
• Preserves the Add AttachDisk/DetachDisk RPCs to VirtualMachineAgent #441 intent. The follow-up handler still operates only on the passed disks and never (re)creates the VM (unlike Invoke/CreateOrUpdate, which can resurrect a concurrently-deleted VM).

Changes

• rpc/cloudagent/compute/virtualmachine/moc_cloudagent_virtualmachine.proto: added the VirtualMachineDiskOperation enum and operation field; replaced the AttachDisk/DetachDisk RPCs with a single UpdateDisks.
• rpc/cloudagent/compute/moc_cloudagent_virtualmachine.pb.go: regenerated with protoc 3.11.4 + protoc-gen-go v1.3.2 (matching the repo toolchain).

Testing

• go build ./... and go vet ./rpc/cloudagent/compute/ pass.
• Handler implementation validated out-of-band in wssdcloudagent (follow-up PR) against this branch.

Follow-up: microsoft/wssdcloudagent will add the UpdateDisks handler and bump its github.com/microsoft/moc dependency to the merged commit of this PR.

Relates to #441.

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