OpenID Connect and default http Client

Nobody has claimed this yet.

In the OpenID Connect provider, it's not possible to change the http Client used to fetch the discovery url.

```go
openidConnect, _ := openidConnect.New(os.Getenv("OPENID_CONNECT_KEY"), os.Getenv("OPENID_CONNECT_SECRET"), "http://localhost:3000/auth/openid-connect/callback", os.Getenv("OPENID_CONNECT_DISCOVERY_URL"))
if openidConnect != nil {
goth.UseProviders(openidConnect)
}
```
Setting `openidConnect. HTTPClient` after new will not work as the call to the Discovery endpoint is done within the new function by calling `openIDConfig, err := getOpenIDConfig(p, openIDAutoDiscoveryURL)`

Happy to work on a PR, but i am not sure how to approach that, i could create a new New with a parameter for the http client, but that would break the pattern and stand out from the other provider. Any suggestion on how to allow to change the HTTPClient there?a package variable in the openid connect provider could work as well, and modification of the Client function

Start with the OpenID Connect provider's New function and the getOpenIDConfig call that fetches the discovery URL, then inspect how HTTPClient and the Client function are defined. Determine an approach that lets callers choose the HTTP client without breaking the provider pattern, and verify that discovery requests use the selected client.

Written by the indexing model from the issue text.