Add factory class to create the process service

[DonJayamanne]

Fixes #1339

This pull request:

• Has a title summarizes what is changing
• Includes a news entry file (remember to thank yourself!)
• Has unit tests & code coverage is not adversely affected (within reason)
• Works on all actively maintained versions of Python (e.g. Python 2.7 & the latest Python 3 release)
• Works on Windows 10, macOS, and Linux (e.g. considered file system case-sensitivity)

[codecov]

Codecov Report

Merging #1342 into master will increase coverage by 0.31%.
The diff coverage is 85.49%.

@@            Coverage Diff             @@
##           master    #1342      +/-   ##
==========================================
+ Coverage   71.35%   71.67%   +0.31%     
==========================================
  Files         274      277       +3     
  Lines       12733    12816      +83     
  Branches     2286     2299      +13     
==========================================
+ Hits         9086     9186     +100     
+ Misses       3507     3496      -11     
+ Partials      140      134       -6

Impacted Files	Coverage Δ
src/client/common/types.ts	100% <ø> (ø)	⬆️
src/client/common/configSettings.ts	87.95% <ø> (ø)	⬆️
src/client/common/process/types.ts	100% <100%> (ø)	⬆️
...t/common/variables/environmentVariablesProvider.ts	100% <100%> (ø)	⬆️
src/client/common/process/proc.ts	100% <100%> (ø)	⬆️
...rc/client/common/process/pythonExecutionFactory.ts	100% <100%> (ø)	⬆️
...ent/interpreter/display/shebangCodeLensProvider.ts	100% <100%> (ø)	⬆️
src/client/sortImports.ts	71.42% <100%> (-2.77%)	⬇️
...ient/interpreter/locators/services/condaService.ts	97.39% <100%> (+0.09%)	⬆️
...ent/interpreter/locators/services/pipEnvService.ts	95.52% <100%> (ø)	⬆️
... and 22 more

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