Improve display of OAuth setup fields

When adding new OAuth providers extension authors can only define the name of the provider which gets displayed on the configuration page along with two fields:

• Key
• Secret

The problem with this approach is that different providers will call these differently making it hard to configure (how to match with what they call a key or secret) plus finding where to configure these providers can be a challenge since there is no way to provide guidance. To improve this situation the provider definitions should be expanded to (optionally) include:

• Provider explanation
• Key label
• Key explanation
• Secret label
• Secret explanation