[Refactor]: Improve configuration loading

[jescalada]

Is your feature request related to a problem? Please describe.
Currently, the app's configuration is set in proxy.config.json, which may cause these problems:

• undefined errors caused by missing entries
• Type errors due to non-normalized schemas for arrays/objects (for example, auth methods all have unique properties, which should be typed and normalized)

Describe the solution you'd like
I think a good solution would be to keep the configuration in a TS file, and load it from there. I believe this would cause breaking changes to current users, so we would have to discuss if bumping up the git-proxy version is possible/desirable or if we could have a hybrid solution instead.

Describe alternatives you've considered
Using environment variables, but that's sadly unrealistic because of the complexity of the config.

Additional context
Working with adding new auth methods (#30, #24) made me realize that this is necessary to make sure users can have auth methods without things blowing up due to missing or malformed config files.