Support drivers and sources in job-configuration schema

[knoepfel]

This PR enforces the schema:

{
  driver: { 
    # Single table of configuration for plugin that registers a driver
    # ...
    
    # N.B. Prototype 0.1 will not support an API for user-definable drivers
    #      Prototype 0.1 includes the 'generate_layers' driver plugin
  },
  sources: { 
    # Table of configurations for plugins that register providers
    # ...
   }, 
  modules: {
    # Table of configurations for modules that register physics algorithms
    # ...
  },
}

---

Closes #106.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
phlex/app/run.cpp	85.71%	1 Missing and 1 partial ⚠️

❌ Your project status has failed because the head coverage (76.25%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

@@            Coverage Diff             @@
##             main     #195      +/-   ##
==========================================
+ Coverage   76.09%   76.25%   +0.15%     
==========================================
  Files         124      124              
  Lines        2719     2737      +18     
  Branches      469      472       +3     
==========================================
+ Hits         2069     2087      +18     
  Misses        453      453              
  Partials      197      197              

Flag	Coverage Δ
unittests	76.25% <92.85%> (+0.15%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
phlex/app/load_module.cpp	79.31% <100.00%> (+4.31%)	⬆️
phlex/core/framework_graph.hpp	100.00% <100.00%> (ø)
phlex/driver.hpp	100.00% <100.00%> (ø)
plugins/generate_layers.cpp	100.00% <ø> (ø)
plugins/python/src/wrap.hpp	100.00% <ø> (ø)
phlex/app/run.cpp	88.23% <85.71%> (-11.77%)	⬇️

... and 1 file with indirect coverage changes

---

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