[Workflow Pause] Proto changes (#653)

<!-- Describe what has changed in this PR -->
Adding proto changes that are required to implement Workflow
Pause/Unpause feature. Added,
 - `WorkflowExecutionStatus.WORKFLOW_EXECUTION_STATUS_PAUSED` enum
- `EVENT_TYPE_WORKFLOW_EXECUTION_PAUSED` &
`EVENT_TYPE_WORKFLOW_EXECUTION_PAUSED` event enums and corresponding
event attributes
- `PauseWorkflowExecution` & `UnpauseWorkflowExecution` gRPC along with
corresponding request/responses.

<!-- Tell your future self why have you made these changes -->
**Why?**
These changes are needed to implement workflow pause/unpause feature.

<!-- Are there any breaking changes on binary or code level? -->
**Breaking changes**
Introduction of new status `WORKFLOW_EXECUTION_STATUS_PAUSED` may break
any saved visibility queries that may have assumed
`WorkflowExecutionStatus != 'Running'` returns all closed workflows.

Author: gow