Make Default Artist Key Configurable

[JPDSousa]

In format parser (AbstractArtistField), a default key is hardcoded as a base key to be used when creating artists from field values.

The problem is that we can only extract the artist name from the field value, whereas there are more required values that cannot be inferred at this stage. This leads to the ArtistKey being "filled" with incorrect values, which goes against the idea of having those fields required.

The only approach here (AFAIK) is to give as much power to the user as we can:

• Let the user define if a default key will be used. If not, the artist inference will always fail, as long as we do not implement a fallback mechanism;
• Let the user define the default values for each key.

Both options will be defined through a Config-based type. Also, note that this is not a muttex situation, as we can perfectly combine both options.