Extra tags for Discogs queries

[jdoe29103]

The Musicbrainz plugin has an extra_tags function that allows querying Musicbrainz with extra tags such as 'catalognum.' This is very useful to me as I have catalog numbers for almost everything in my library so getting 100% is a breeze. However, the Discogs plugin does not have this function. As a result I have many albums that have 100% matches on Discogs but can only be found by greatly increasing my search limit.

For example I have DGC – UICY-75129 which has over 150 versions on Discogs. If I increase my search_limit to 170 I am able to get the 100% no problem but if not, the match never shows up because Discogs is not being queried with 'catalognum'

This has already somewhat been addressed in #4926 but I am not confident in my Python abilities to implement the solution myself. I think I could take a stab at this if someone more familiar with the Musicbrainz implementation could layout what changes would need to be made.