[Refactor]: separate context impl and interface

[zhongkechen]

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Description

• Added interfaces for DurableContext, StepContext and BaseContext
• Separate implementation of the interfaces to impl classes
• fix of ctx.isReplaying [Bug]: fix isReplaying in BaseContext #222

This is to prevent users from misuse of some implementation only methods such as getExecutionManager()

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• I have thoroughly tested this change

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