Banshee.IO: move Provider's schema field into new class

To avoid GSettingsSchemaExtractor calling the static constructor of Provider
class and crashing, we put this SchemaEntry into a new class.

The exception was, for the record:

Found SchemaEntry: Banshee.IO.Provider.ProviderSchema

Unhandled Exception:
System.TypeInitializationException: An exception was thrown by the type initializer for Banshee.IO.Provider ---> System.InvalidOperationException: Add-in engine not initialized.
  at Mono.Addins.AddinEngine.CheckInitialized () [0x00000] in <filename unknown>:0
  at Mono.Addins.AddinManager.GetExtensionNodes (System.String path) [0x00000] in <filename unknown>:0
  at Banshee.IO.Provider.GetOrderedExtensions (System.String extensionPoint, System.String[] ordered_ids) [0x00000] in <filename unknown>:0
  at Banshee.IO.Provider..cctor () [0x00000] in <filename unknown>:0
  --- End of inner exception stack trace ---
  at (wrapper managed-to-native) System.Reflection.MonoField:GetValueInternal (System.Reflection.MonoField,object)
  at System.Reflection.MonoField.GetValue (System.Object obj) [0x00000] in <filename unknown>:0
  at GSettingsSchemaExtractorProgram.Extract (IEnumerable`1 types) [0x00000] in <filename unknown>:0
  at GSettingsSchemaExtractorProgram.Extract (System.IO.DirectoryInfo dir) [0x00000] in <filename unknown>:0
  at GSettingsSchemaExtractorProgram.Main (System.String[] args) [0x00000] in <filename unknown>:0