chore: Add attoseconds accessor (#3)

* Add attoseconds accessor.

* Add initializer.

Sources/DateTime/InternalUTCClock.swift

@@ -29,10 +29,23 @@ public struct InternalUTCClock {
         internal init(_value: Swift.Duration) {
             self._value = _value
         }
-
+
+        public init(secondsComponent: Int64, attosecondsComponent: Int64) {
+            self._value = Swift.Duration(secondsComponent: secondsComponent, attosecondsComponent: attosecondsComponent)
+        }
+
+        @available(*, deprecated, renamed: "seconds")
         public func secondsSinceEpoch() -> Int64 {
             return self._value.components.seconds
         }
+
+        public func seconds() -> Int64 {
+            return self._value.components.seconds
+        }
+
+        public func attoseconds() -> Int64 {
+            return self._value.components.attoseconds
+        }
     }
 
     public init() {}