feat: allow reading secrets from files

[Gregofi]

Allows reading password from file whose path is in environment variable.

[Gregofi]

I tried to make this struct as reusable as possible. If another secret was added in the future, it can be reused without any hassle.

The reason for env:"" (and not env:"PASSWORD") is that it doesn't lock into naming (for example if authentication token was ever added, it can be named LOGIN_TOKEN, not something ending with PASSWORD).

The drawback of this solution is that the fields are now public. They were accessible before via the getter, but now one must be even more careful to not print them (for example by explicitly implementing serializers, as below).

[davidmasek]

hi, thanks for PR.

It is not passing the "Docker Image CI" test. Not sure if the problem is with the changes or the CI pipeline, I'll take a proper look later.

When logging the parsed output the "email" section is empty -
https://github.com/davidmasek/beacon/actions/runs/12837955985/job/35814332202?pr=20#step:3:340

email:
    smtp_server: ""
    smtp_port: 0
    smtp_username: ""
    smtp_password: '*****'
    send_to: ""
    sender: ""
    prefix: '[staging]'
    enabled: ""

[davidmasek]

The testing is currently broken when the PR originates from a fork. I will track the bug here - #22 .

Thanks for help, I'm merging this manually