Allow a flow definition's identifier and namespace to be specified within the XML flow definition itself

Currently you cannot set a flow identifier explicitly in your XML-based flow definition. You either rely on id assignment by convention, or manually specify the id when the flow is registered in a flow definition registry. The simplest, most object oriented approach is to allow the flow to identify itself. This should be supported, without removing the support we have for externalized id and package assignment [which provides added flexibility].

Examples

<flow namespace="account" id="show">
...
</flow>

Alternative approach:

<flow id="account.show">

</flow>