[chore] Apply improvements to `Config` class

This MR is part of a larger effort to support:

(1) the migration to JSON-based config parsing; (2) safer, more informative exception handling in SCDAQ.

Changelog summary:

• Implements wrapper class around the std::map with the configurations read by the boost-based parsing step.
• Addresses linter warnings, e.g., typos, annotations, const-ness.
• Suppresses string copying when not needed.
• Renames data members and classes to align with Google's C++ style guide: https://google.github.io/styleguide/cppguide.html#Variable_Names, e.g.:
  • class config --> class Config
  • memberVariable --> member_variable_