[#59] feat(bin): add default configuration file paths

[M0dEx]

This PR adds default configuration file paths for all compiled binaries.

The default are:

• client.toml for the client binary
• server.toml for the server binary
• users for the users binary

Closes #59.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 82.01%. Comparing base (55c35dc) to head (3ac2df8).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #61 +/- ## ========================================== + Coverage 81.23% 82.01% +0.78% ========================================== Files 17 17 Lines 906 901 -5 ========================================== + Hits 736 739 +3 + Misses 170 162 -8 ```

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