Move config.hpp out of include folder

[thirtytwobits]

When pulling libcyphal in as a submodule or cmake external project it's very difficult to exclude the default config.hpp from include paths to allow it to be overridden by the application. We should provide the default in a different folder and allow the application to either use it by including that path or to create their own.

See https://github.com/FreeRTOS/FreeRTOS-Kernel/blob/main/examples/template_configuration/readme.md for an example of a project that does this.

[thirtytwobits]

We should also, always include this header as libcyphal/config.hpp or we'll want to rename it to libcyphal_config.hpp

[emrainey]

Definitely need a way to exclude the default config and have your own. Separate directories make it easy to select.

[serges147]

a way to exclude the default config

Actually it was deliberate design idea to have a default config - so that if you need custom one you don't have to define EVERYTHING from scratch. Instead, you inherit your custom config from the default one, and "override" only those constants which need to be tuned. Please see how it's done/tested at test/unittest/custom_libcyphal_config.hpp and test_publisher_custom_config.cpp.