Add possibility to remove the RetryPolicy from the workflow context without overriding the other existing ActivityOptions in it

[kittinanasi]

The WithRetryPolicy function accepts a pointer receiver RetryPolicy parameter, so it can be used to remove the RetryPolicy from the ActivityOptions.

I set the ActivityOptions including a non-nil RetryOptions configuration at the beginning of my workflow, these ActivityOptions are used by all activities in my workflow by default. I would like to override the RetryPolicy to nil for just one activity in that workflow without overriding the other ActivityOptions configurations.

I added a unit test.

When users set a retryPolicy with the WithRetryPolicy function, it could be broken, so the workflow might not be retried even if the RetryPolicy is set.