Add history_size_bytes to WorkflowExecutionInfo and PollWorkflowTaskQueueResponse

[mfateev]

Is your feature request related to a problem? Please describe. Controlling a workflow history size is important to keep the system performant. None of the existing APIs exposes the aggregated size of the history.

Describe the solution you'd like Add history_size_bytes to WorkflowExecutionInfo (returned from DescribeWorkflowExecution) and PollWorkflowTaskQueueResponse.

[yiminc]

Should also expose history size to workflow task started event so workflow code can access this info as well as they might use it to decide if continue as new is needed.