Claims parameter should be configured as a dictionary

[miwig]

OIDC spec:

claims
    OPTIONAL. This parameter is used to request that specific Claims be returned.  
    The value is a JSON object listing the requested Claims. 

We can specify this in the config as a yaml dict (to avoid ugly escaped-json-in-a-string) and serialize it into a json string for the request. serde_json::Map or (if must be) serde_json::Value may be appropriate types for this.

[antonengelhardt]

Maybe as part of #99 to avoid more breaking changes