Add getUnchainedWorkflowContext to WorkflowImpl

Problem:
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• In the WorkflowImpl class there are shorthand methods for obtaining the work-flow context
• To get an unchained work-flow context developers need to make a call to getWorkflowContext(null), notice the 'null' parameter
• This is not really readable, and it would be better to have another shorthand method which explicitly explains the role of such call

Solution:
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• Add a shorthand method to the WorkflowImpl: getUnchainedWorkflowContext