Options to ignore already started error

[longquanzheng]

It's been a common pattern to ignore AlreadyStartedError.

See example in Temporal Golang SDK's StartWorkflowOptions

Implementation idea:

• Option 1: pass in the ignore as a flag in the start request. So that the server will respond 200 status. (preferred)
• Option 2: Alternatively, doing it only in SDK is also fine. But server will respond 400 status today, which will look a bit annoying.

Another big reason for option 1 is that we can support passing in a idempotent key(uuid) to dedup(already supported in cadence temporal). Only throw error if the key not match.

This dedup key is super useful for case like parent starts child workflows from a channel message. Where msg can be duplicated. It can use state execution id as dedup key.