add possibility to configure http client for oidc provider

[ostempel]

Description

For our mini-lab we need the possibility to skip tls verification. Therefore we need to manipulate the http-transporter of the oidc-provider. Otherwise it will fail because of unknown CA and self-signed certs.

The http-client of the provider gets accessed through:

goth/providers/openidConnect/openidConnect.go

Lines 198 to 200 in 15b24f5

	func (p *Provider) Client() *http.Client {
	return goth.HTTPClientWithFallBack(p.HTTPClient)
	}

And the Provider wraps this with the DefaultClient if it is not set.

goth/provider.go

Lines 82 to 87 in 15b24f5

	func HTTPClientWithFallBack(h *http.Client) *http.Client {
	if h != nil {
	return h
	}
	return http.DefaultClient
	}