Add support for missing SecuritySchemes

While a number of [SecuritySchemes](https://w3c.github.io/wot-thing-description/#sec-security-vocabulary-definition) 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](https://w3c.github.io/wot-thing-description/#combosecurityscheme) ~(Feature is currently marked as "at risk")~

### HTTP Schemes

* [ ] Implement [OAuth2SecurityScheme](https://w3c.github.io/wot-thing-description/#oauth2securityscheme)
* Dart's [oauth2](https://pub.dev/packages/oauth2) can be used to implement this
* [ ] Implement [APIKeySecurityScheme](https://w3c.github.io/wot-thing-description/#apikeysecurityscheme)

### CoAP Schemes

* [x] Implement [PSKSecurityScheme](https://w3c.github.io/wot-thing-description/#psksecurityscheme)
* [x] #8