Validate external improvements

[egillax]

• Add logging for population creation
• Error when validating and outcomes are <10
• create downloadTasks list from unique combinations of restrictPlpDataSettings and targetId to avoid downloading the same thing multiple times

[codecov]

Codecov Report

Attention: Patch coverage is 85.10638% with 14 lines in your changes missing coverage. Please review.

Project coverage is 86.98%. Comparing base (5f53965) to head (2ab8d7c).

Files with missing lines	Patch %	Lines
R/ExternalValidatePlp.R	84.61%	14 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #485      +/-   ##
===========================================
+ Coverage    86.09%   86.98%   +0.88%     
===========================================
  Files           50       50              
  Lines        10286    10369      +83     
===========================================
+ Hits          8856     9019     +163     
+ Misses        1430     1350      -80     

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