Add an OAuth2Client class

[porcellus]

We want the interface to be similar to the response type of https://www.ory.sh/docs/hydra/reference/api#tag/oAuth2/operation/getOAuth2Client

If you'd like you can make changes to how the properties are organized (e.g.: create sub-objects), but then please add a deserialization function as well (fromJSON)

Please use feat/oauth2/base as a base&target branch.