Collect Credential settings from server in attributes hash rather than being explicit.

[plukevdh]

Credentials change based on the plugin type. For example JWT vs basic auth have a different set of keys. This makes mapping them explicitly very awkard.

https://github.com/articulate/biplane/blob/master/src/biplane/models/credential.cr#L18-L24

Rewrite the initializer (or from_json method?) to collect unused attrs in this fashion into an attributes hash. Should also simplify compares of things like this.

May affect other plugins as well...