Add generic OpenID Connect provider (OAuth2)

This adds the oauth2 provider `oidc`. It needs an additional argument, the OIDC discovery endpoint to figure out where the auth and token URLs are.

Configuration is similar to setting up the Google Authentication with these changes:

 * `OAUTH2_PROVIDER = oidc`
 * `OAUTH2_OIDC_DISCOVERY_ENDPOINT = https://auth.exampe.org/discovery`

config/config_test.go

@@ -832,6 +832,41 @@ func TestDefaultOAuth2RedirectURLValue(t *testing.T) {
 	}
 }
 
+func TestOAuth2OidcDiscoveryEndpoint(t *testing.T) {
+	os.Clearenv()
+	os.Setenv("OAUTH2_OIDC_DISCOVERY_ENDPOINT", "http://example.org")
+
+	parser := NewParser()
+	opts, err := parser.ParseEnvironmentVariables()
+	if err != nil {
+		t.Fatalf(`Parsing failure: %v`, err)
+	}
+
+	expected := "http://example.org"
+	result := opts.OAuth2OidcDiscoveryEndpoint()
+
+	if result != expected {
+		t.Fatalf(`Unexpected OAUTH2_OIDC_DISCOVERY_ENDPOINT value, got %q instead of %q`, result, expected)
+	}
+}
+
+func TestDefaultOAuth2OidcDiscoveryEndpointValue(t *testing.T) {
+	os.Clearenv()
+
+	parser := NewParser()
+	opts, err := parser.ParseEnvironmentVariables()
+	if err != nil {
+		t.Fatalf(`Parsing failure: %v`, err)
+	}
+
+	expected := defaultOAuth2OidcDiscoveryEndpoint
+	result := opts.OAuth2OidcDiscoveryEndpoint()
+
+	if result != expected {
+		t.Fatalf(`Unexpected OAUTH2_REDIRECT_URL value, got %q instead of %q`, result, expected)
+	}
+}
+
 func TestOAuth2Provider(t *testing.T) {
 	os.Clearenv()
 	os.Setenv("OAUTH2_PROVIDER", "google")