Add more state to the pending activity state (#565)

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<!-- Describe what has changed in this PR -->
**What changed?**
Add PAUSED/PAUSE_REQUESTED states to the PendictActivityState enum.

<!-- Tell your future self why have you made these changes -->
**Why?**
To easily differentiate between "activity is paused on server and still
running on worker" and "activity is paused on server and not running on
worker". This is the result of our discussion for activity commands.

In old terms:
"state == PAUSED" is "state = SCHEDULLED and paused_flag = true"
"state == PAUSE_REQUESTED" is "state = STARTED and paused_flag = true"

Author: ychebotarev