refactor: rework config package

[lucasoares]

This PR makes the configuration more readable and easier to use

It also changes to use gotestsum instead of go test to provide better readability of test results.

[codecov]

Codecov Report

Merging #18 (2274a05) into main (d2b2f6d) will increase coverage by 0.06%.
The diff coverage is 90.54%.

@@            Coverage Diff             @@
##             main      #18      +/-   ##
==========================================
+ Coverage   80.15%   80.21%   +0.06%     
==========================================
  Files          22       24       +2     
  Lines        3330     3315      -15     
==========================================
- Hits         2669     2659      -10     
+ Misses        540      536       -4     
+ Partials      121      120       -1     

Impacted Files	Coverage Δ
internal/messagepool/message_pool_housekeeper.go	80.38% <ø> (ø)
internal/messagepool/storage/mongo_storage.go	78.23% <76.92%> (+0.04%)	⬆️
internal/config/config.go	86.95% <78.57%> (-5.91%)	⬇️
internal/audit/audit.go	86.55% <85.71%> (ø)
internal/cmd/deckard/main.go	49.66% <100.00%> (ø)
internal/config/logging.go	100.00% <100.00%> (ø)
internal/config/viper_config.go	100.00% <100.00%> (ø)
internal/logger/logger.go	71.92% <100.00%> (+7.01%)	⬆️
internal/messagepool/cache/redis_cache.go	88.15% <100.00%> (ø)
internal/service/deckard_service.go	79.64% <100.00%> (ø)