Introduce paths according to XDG specification in `iceoryx_platform`

## Brief feature description

Files required for unit tests, the iceoryx config file, shared memory file handles or other user defined files have no well defined place to be currently in iceoryx.
When we define constants like `constexpr const char* TEMP_DIR = "/tmp";` in the `platform_settings.hpp` file we could define custom paths for every platform and use them for storage of test files, to look up the iceoryx config file or other things.

Paths we currently require:
* iceoryx_config
* system wide config lookup path
* user config lookup path
* unit tests
* path for temporary files

Start with platform_settings.hpp and trace how iceoryx currently locates its config, creates unit-test temporary files, and handles user-defined files. Compare the required system-wide and user config paths and temporary-file path with the XDG specification across supported platforms. Done means the paths are defined per platform and used consistently for these storage and lookup needs, with unit tests covering the behavior.

Written by the indexing model from the issue text.