Rename ValidHandlerStatus to ExtensionStatusValue

[narrieta]

During a previous PR we talked about renaming ValidHandlerStatus to ExtensionStatus. There is already an ExtensionStatus in the restapi module so using ExtensionStatusValue instead.

For consistency, I also renamed HandlerStatus to ExtHandlerStatusValue (since there is a ExtHandlerStatus in restapi)

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (develop@8b9a2b1). Click here to learn what that means.
The diff coverage is 100.00%.

❗ Current head f6844ed differs from pull request most recent head 9060ff1. Consider uploading reports for the commit 9060ff1 to get more accurate results

@@            Coverage Diff             @@
##             develop    #2376   +/-   ##
==========================================
  Coverage           ?   70.94%           
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  Files              ?       97           
  Lines              ?    14107           
  Branches           ?     2029           
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  Hits               ?    10008           
  Misses             ?     3654           
  Partials           ?      445           

Impacted Files	Coverage Δ
azurelinuxagent/common/logcollector_manifests.py	100.00% <ø> (ø)
azurelinuxagent/ga/exthandlers.py	86.25% <100.00%> (ø)
azurelinuxagent/ga/update.py	88.31% <100.00%> (ø)

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