Manage FhirRequestBuilders via ManagedWebAccess

[dotasek]

Rework ManagedWebAccess and the various FhirRequestBuilders to ensure the following:

• FhirRequestBuilders and SimpleHTTPClient should always use ManagedWebAccess to:
  • Set the 'User-Agent'
  • Set appropriate 'Authorization' or 'Api-Key' headers

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 12.73%. Comparing base (6775a09) to head (765ba8b).
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1788      +/-   ##
============================================
+ Coverage     12.71%   12.73%   +0.02%     
- Complexity    32947    33028      +81     
============================================
  Files          2211     2218       +7     
  Lines        675662   675849     +187     
  Branches     199169   199208      +39     
============================================
+ Hits          85881    86059     +178     
+ Misses       559303   559294       -9     
- Partials      30478    30496      +18     

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[dotasek]

Merging after Zulip confirmation from @grahamegrieve