Provide user identity on user-initiated events #8538
Description
Describe the solution you'd like
Need something like HistoryEvent.user_identity that is set for any events that are initiated by users (so signals and cancellation requests and such, but not timer creation/firing). Up to implementer on details like field name and whether it's optional on top-level events or copied to many specific events. This should only be settable by server as the literal user identity which is assumed to be in context and work in a common way for OSS authorizers and cloud users. This is completely unrelated to existing identity (aka "client identity" or "worker identity") fields.