feat/refactor: prevent config deletion when a new field is added

[lorenzoberts]

This PR aims to solve issue #118 .

The first commit is a minor refactor to improve code quality in the app/config.rs module. It removes redundancies, centralizes the config file name construction and implements the std::Default trait (the default function was already there; i just had to move it to the trait impl).

The second commit solves the issue itself, but without any "optimizations". As @davidbtadokoro mentioned in the issue description, we just had to create a #[serde(default = "")] attribute for each Config field. We could merge only the first two commits, and the main problems would have been solved. Despite that, with this solution we've created two other problems:

1. For each field's default function, we need to reinitialize the Config::default() object. This means accessing env vars and creating strings from scratch for every field that cannot be deserialized automatically.
2. For each new field we add to Config, we would have to also add the serde(default = "") attribute and the default implementation, which is easy but not quite clean, making the structure hard to read and the config.rs code too extensive.

The third commit solves problem 1 by applying lazy_static macro, which creates the Config::default object only once at runtime.

The fourth commit solves problem 2 by creating a procedural macro, which will do the exact same thing commit 2 and 3 (combined) did, but it is extensible for all eventual new fields the struct Config will receive.
I found it a good opportunity to create the first procedure macro for patch-hub but, as any macro, it only solves developers' pains, so if you think it's overkill we can stop the issue resolution on commit 3.

The fifth commit creates unit tests for the procedural macro and it changes the CI workflow so it is can run those tests.

[davidbtadokoro]

Hey @lorenzoberts, and thank you so much for this PR! I have a list of things to say, so here are some bullet points:

• It indeed solves issue patch-hub config file gets deleted when adding new field to Config #118. Tested it through and through, and it didn't break either in intermediary commits;
• I loved how you didn't stop in solving the issue, but also thought about the maintainability of your solution. I had to spend some time in learning procedural macros to adequately review this PR, which is good for knowledge, and also made me agree with this approach to avoid having to have a lot of boilerplate and annoyance when reading/changing app/config.rs (or future modules with the same behavior);
• All commits were perfect, IMHO, and I had virtually no tinkers, besides a preliminary commit (next bullet). Kudos for the messages and clean implementation (I struggled a little with the macro, but it turned out to be really clear 😺).
• Because we are now altering the config file when calling Config::build() - which I agree to enforce consistency of the loaded and consolidated files - we had a side-effect of altering the sample config file src/test_samples/app/config/config.json. This makes running the test suite alter this git tracked file, so I added a preliminary commit to make the test suite use a temporary sample config file (with the same contents as the tracked one) to avoid this and not change your solution.

Long story short, thank you so much for this marvelous PR, and sorry for the delay. Change merged into the unstable branch 👍