fix: config schema

[msagi]

Fix for #509.

• Updated the schema which did not seem to be correct as per the schema spec ("oneOf" should go under "properties" not the other way around).
• Moved config file validation logic into the config file module for improved encapsulation
• Added a few positive and negative test scenarios for config files

@JamieSlome: should GitProxy always validate the config file when starting? It seems it only runs the validation if -v/--validate is present.

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 64.69%. Comparing base (71f84e5) to head (cf6aa75).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #526      +/-   ##
==========================================
+ Coverage   64.23%   64.69%   +0.45%     
==========================================
  Files          40       40              
  Lines        1191     1198       +7     
==========================================
+ Hits          765      775      +10     
+ Misses        426      423       -3     

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[JamieSlome]

@msagi - I think we should make it such that we validate the config prior to starting the service.

[JamieSlome]

@msagi - can we create a fixtures folder in the test folder to store all of the mock data?