Make extension manifests optional

[narrieta]

The extension manifest can be omitted in the goal state when, for example, uninstalling an extension that has been removed from storage. In that case, the manifest list should be empty .

[codecov]

Codecov Report

Merging #2537 (dc5cdc3) into develop (5cf4bb3) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #2537      +/-   ##
===========================================
+ Coverage    71.99%   72.01%   +0.02%     
===========================================
  Files          102      102              
  Lines        15194    15196       +2     
  Branches      2403     2404       +1     
===========================================
+ Hits         10939    10944       +5     
- Misses        3773     3774       +1     
+ Partials       482      478       -4     

Impacted Files	Coverage Δ
...protocol/extensions_goal_state_from_vm_settings.py	83.51% <100.00%> (+0.49%)	⬆️
azurelinuxagent/common/event.py	85.36% <0.00%> (-0.68%)	⬇️
azurelinuxagent/ga/collect_telemetry_events.py	91.43% <0.00%> (+1.71%)	⬆️

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