feat(spdk): support backing image in spdk

[ChanYiLin]

ref: longhorn/longhorn#6341

• Add BackingImageName in SpdkInstanceSpec, so it can create spdk replica with backing image
• Add proxy api, longhorn-manager will access spdk server through the engine proxy, but it don't have engine involved.

    rpc SPDKBackingImageCreate(SPDKBackingImageCreateRequest) returns (SPDKBackingImageResponse);
    rpc SPDKBackingImageDelete(SPDKBackingImageDeleteRequest) returns (google.protobuf.Empty);
    rpc SPDKBackingImageGet(SPDKBackingImageGetRequest) returns (SPDKBackingImageResponse);
    rpc SPDKBackingImageList(google.protobuf.Empty) returns (SPDKBackingImageListResponse);
    rpc SPDKBackingImageWatch(google.protobuf.Empty) returns (stream google.protobuf.Empty) {}

• Add spdk api, so we can manage backing image in spdk server

    rpc BackingImageCreate(BackingImageCreateRequest) returns (BackingImage);
    rpc BackingImageDelete(BackingImageDeleteRequest) returns (google.protobuf.Empty);
    rpc BackingImageGet(BackingImageGetRequest) returns (BackingImage);
    rpc BackingImageList(google.protobuf.Empty) returns (BackingImageListResponse);
    rpc BackingImageWatch(google.protobuf.Empty) returns (stream google.protobuf.Empty) {}
    rpc BackingImageExpose(BackingImageGetRequest) returns (BackingImageExposeResponse) {}
    rpc BackingImageUnexpose(BackingImageGetRequest) returns (google.protobuf.Empty) {}

[derekbit]

After fixing the indentation, we can merge the PR.

[derekbit]

LGTM