add workflow context and add history event iterator

[shijiesheng]

What changed?

• moved workflow package into internal
• added workflow context that will activate on process_decision function
• added a generator for HistoryEvent which will read pages to get full history. It's a util function

Why?

How did you test it?

Potential risks

Release notes

Documentation Changes

[timl3136]

nit: importing this context from cadence._internal.workflow.context import Context will confuse people with from cadence.workflow import WorkflowContext

[shijiesheng]

i think that's a misunderstanding. This is just an internal implementation that user will never get exposed to.