Add support for more timeouts to Nexus operations

[Quinn-With-Two-Ns]

Add schedule-to-start and start-to-close for Nexus operations

NOTE, before this can be merged the API still needs to be tagged, core changes need to be merged and tests need to be added

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Note

Nexus operations

• Extend ScheduleNexusOperationCommandAttributes with schedule_to_start_timeout and start_to_close_timeout (read/write/merge + descriptors)

Continue-as-new

• Add initial_versioning_behavior to ContinueAsNewWorkflowExecutionCommandAttributes

Activities

• New ActivityExecutionOutcome (result/failure oneof)
• Add ActivityExecutionInfo and ActivityExecutionListInfo message types
• Extend ActivityOptions with priority

Common payloads

• Add Payload.ExternalPayloads (with nested ExternalPayloadDetails.size_bytes) for externally stored payload metadata

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