Move SourceType to an Enum

[Trenly]

Description of the new feature / enhancement

Currently source types are all either std::string or std::string_view in the code. To me, it seems like this isn't a very good way of ensuring the source types are well managed

Proposed technical implementation details

Refactor the code to use a SourceType enum with a method for converting to SourceType from a string or string_view, and a method for converting SourceType to string view. This can then be used for stronger argument validation on the source add command