Prepare configs for db and filesystem

[piotr-iohk]

In preparation for introducing database and filesystem storage into uptime-service-backend this pr:

• introduces into app_config.json database and filesystem sections with related configs entries (specific fields inside will probably change, as this is mainly to introduce the mechanism for reading those multiple configs)
• merges aws_creds.json into app_config.json such that aws credentials live in the aws section in the app_config.json
• introduces unit tests for LoadEnv which loads the configs
• updates integration tests accordingly to use new configs

[Smorci]

A more generic question: how did you test your code? What results did you get?

[piotr-iohk]

A more generic question: how did you test your code? What results did you get?

Added unit tests for LoadEnv that loads configurations. Also tested via integration test that AWS S3 backend still works as before.

[Sventimir]

Looks good to me 👍

There are some minor issues, but nothing blocking, I think.

[Smorci]

All good on my end!