refresh value from key vault reference

[Eskibear]

Improvements:

• If version of a resolved kvr is not changed, we don't need to re-resolve the value again.
• If version of a kvr is "latest", even though the kvr stored in app config store is not changed, the version of the kvr can be updated. We should try resolving the value if latest version is changed.

[zhiyuanliang-ms]

#175

[zhiyuanliang-ms]

Released in 2.2.0