message: breaking.md changes related to replacing isAttached with attachState
diff: - [Add Undefined to getAbsoluteUrl return type](#Add-Undefined-to-getAbsoluteUrl-return-type)
  - [Renamed TestDeltaStorageService, TestDocumentDeltaConnection, TestDocumentService, TestDocumentServiceFactory and TestResolver](#Renamed-TestDeltaStorageService,-TestDocumentDeltaConnection,-TestDocumentService,-TestDocumentServiceFactory-and-TestResolver)
  - [DocumentDeltaEventManager has been renamed and moved to "@fluidframework/test-utils"](#DocumentDeltaEventManager-has-been-renamed-and-moved-to-"@fluidframework/test-utils")
  +- [`isAttached` replaced with `attachState` property](#`isAttached`-replaced-with-`attachState`-property)
  ### Deprecated `path` from `IComponentHandleContext`
  Deprecated the `path` field from the interface `IComponentHandleContext`. This means that `IComponentHandle` will not have this going forward as well.
  @@ -134,6 +135,13 @@ opProcessingController = new OpProcessingController(deltaConnectionServer);
  opProcessingController.addDeltaManagers(component1.runtime.deltaManager, component2.runtime.deltaManager);
  ```
  +### `isAttached` replaced with `attachState` property
  +
  +`isAttached` is replaced with `attachState` property on `IContainerContext`, `IContainerRuntime` and `IComponentContext`.
  +`isAttached` returned true when the entity was either attaching or attached to the storage.
  +So if `attachState` is `AttachState.Attaching` or `AttachState.Attached` then `isAttached` would have returned true.
  +Attaching is introduced in regards to Detached container where there is a time where state is neither AttachState.Detached nor AttachState.Attached.
  +
  ## 0.21 Breaking Changes
  - [Removed `@fluidframework/local-test-utils`](#removed-`@fluidframework/local-test-utils`)
  - [IComponentHTMLVisual deprecated](#IComponentHTMLVisual-deprecated)
  