feat(server): expose main credit stat to reflect only album artist | artist credit (#4268)

* attempt using artist | albumartist

* add primary stats, expose to ND and Subsonic

* response to feedback (1)

* address feedback part 1

* fix docs and artist show

* fix migration order

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Co-authored-by: Deluan Quintão <deluan@navidrome.org>

model/participants.go

@@ -25,6 +25,8 @@ var (
 	RoleRemixer     = Role{"remixer"}
 	RoleDJMixer     = Role{"djmixer"}
 	RolePerformer   = Role{"performer"}
+	// RoleMainCredit is a credit where the artist is an album artist or artist
+	RoleMainCredit = Role{"maincredit"}
 )
 
 var AllRoles = map[string]Role{
@@ -41,6 +43,7 @@ var AllRoles = map[string]Role{
 	RoleRemixer.role:     RoleRemixer,
 	RoleDJMixer.role:     RoleDJMixer,
 	RolePerformer.role:   RolePerformer,
+	RoleMainCredit.role:  RoleMainCredit,
 }
 
 // Role represents the role of an artist in a track or album.