Add support for missing SecuritySchemes

[JKRhb]

While a number of SecuritySchemes have already been implemented, there are still a number of Schemes missing. This issue serves as a tracker for the remaining Schemes which can be devided into a generic one (the ComboSecurityScheme) and (mostly) HTTP or CoAP related ones.

Generic Schemes

• Implement ComboSecurityScheme (Feature is currently marked as "at risk")

HTTP Schemes

• Implement OAuth2SecurityScheme
  • Dart's oauth2 can be used to implement this
• Implement APIKeySecurityScheme

CoAP Schemes

• Implement PSKSecurityScheme
• Implement ACESecurityScheme #8