Move WorkflowExecutionPauseInfo inside WorkflowExecutionExtendedInfo (#666)

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This PR moves `pause_info` into `WorkflowExecutionExtendedInfo`.

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In temporalio/api#653 I accidentally put
pause_info in `WorkflowExecutionInfo`. It should have been in
`WorkflowExecutionExtendedInfo`

**Note**: This was merged yesterday (and the corresponding server
changes are not yet merged). There are no references or usage of the
`WorkflowExecutionInfo.pause_info` field. So was not necessary to
reserve the proto field.

Author: gow