Not clear how to pass constant query params to authorize_url
The atlassian oauth provider requires you to always pass `audience=api.atlassian.com&response_type=code&prompt=consent` to the "authorize url".

Now you could set `custom_parameters` to `['audience', 'response_type', 'prompt']` via `oauth.json` and then the user would always need to set `custom_params` to `{audience: 'api.atlassian.com', response_type: 'code', prompt: 'consent' }` in their configuration, but it doesn't seem worthwhile to make the user do this.

Does grant support a way to encode these constants in `oauth.json`? I looked at the existing file and nothing seems clear. If it does support this, could the documentation be updated to make it clear how to do this?

Thanks!