Separate provider code into modules

[ionut-arm]

Currently all providers are implemented as "monolithic modules" - i.e. all functionality is implemented in one large file.

For ease of use it would be much better to split them and put each type of operation/method into it own file, e.g. sign.rs, key_management.rs etc.

This should be done for all providers.

All utility methods (e.g. for managing interaction with the KeyIDManager) should reside in their own module(s) as well.