Add saga pattern support in Dapr workflow

[skyao]

This document is a design proposal to support the Saga pattern in Dapr workflow.

This proposal will allow applications to support distributed transactions using Dapr Workflow and its Saga pattern support feature. It will be helpful when there are consistency requirements for some activities in the workflow.

will close #47.

Related:

• PR of proposal content: Add saga pattern support in Dapr workflow #47
• PR of code changed in dapr/java-sdk repo: Add Saga pattern support in java-sdk java-sdk#956
• PR of new Saga quickstart in dapr/quickstarts repo: Add Saga pattern quickstart in Java quickstarts#957