Add OnConflictOptions to StartWorkflowExecutionRequest (#510)

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<!-- Describe what has changed in this PR -->
**What changed?**
Add `OnConflictOptions` to `StartWorkflowExecutionRequest`

<!-- Tell your future self why have you made these changes -->
**Why?**
When the workflow id conflict policy is `USE_EXISTING`,
`OnConflictOptions` add the ability to change the existing running
workflow.

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**Breaking changes**

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right after this PR was merged. -->
**Server PR**
temporalio/temporal#7080