[#130] sdk: Define object types

Define Object type that represents the immutable NeoFS object. Define
RawObject type that provides interface for convenient construction of NeoFS
object.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/object/raw.go

@@ -0,0 +1,53 @@
+package object
+
+import (
+	"crypto/sha256"
+
+	"github.com/nspcc-dev/neofs-api-go/pkg/container"
+	"github.com/nspcc-dev/neofs-api-go/pkg/owner"
+	"github.com/nspcc-dev/neofs-api-go/v2/refs"
+)
+
+// RawObject represents NeoFS object that provides
+// a convenient interface to fill in the fields of
+// an object in isolation from its internal structure.
+type RawObject struct {
+	rwObject
+}
+
+func (o *RawObject) set(setter func()) {
+	o.fin = false
+	setter()
+}
+
+// SetContainerID sets object's container identifier.
+func (o *RawObject) SetContainerID(v *container.ID) {
+	if o != nil {
+		o.set(func() {
+			o.cid = v
+		})
+	}
+}
+
+// SetOwnerID sets identifier of the object's owner.
+func (o *RawObject) SetOwnerID(v *owner.ID) {
+	if o != nil {
+		o.set(func() {
+			o.ownerID = v
+		})
+	}
+}
+
+// SetPayloadChecksumSHA256 sets payload checksum as a SHA256 checksum.
+func (o *RawObject) SetPayloadChecksumSHA256(v [sha256.Size]byte) {
+	if o != nil {
+		o.set(func() {
+			if o.payloadChecksum == nil {
+				o.payloadChecksum = new(refs.Checksum)
+			}
+
+			o.payloadChecksum.SetType(refs.SHA256)
+			o.payloadChecksum.SetSum(v[:])
+		})
+	}
+}