Prepare to remove go-oidc/oauth2 from config packages

Adds a OAuthClientCredentials type directly into the servicecfg
package so that the dependency on go-oidc/oauth2 can be removed.
The PluginOAuthProviders.Slack field has been deprecated, but left
in place so as not to break teleport.e and a new
PluginOAuthProviders.SlackCredentials field has been added to use
the new type. Both fields are being populated with the same data
from the file config at the moment. Once teleport.e has been updated
to consume the credentials from PluginOAuthProviders.SlackCredentials
the original field will be removed.

lib/config/configuration_test.go

@@ -3673,6 +3673,10 @@ func TestAuthHostedPlugins(t *testing.T) {
 				require.NotNil(t, p.OAuthProviders.Slack)
 				require.Equal(t, "foo", p.OAuthProviders.Slack.ID)
 				require.Equal(t, "bar", p.OAuthProviders.Slack.Secret)
+
+				require.NotNil(t, p.OAuthProviders.SlackCredentials)
+				require.Equal(t, "foo", p.OAuthProviders.SlackCredentials.ClientID)
+				require.Equal(t, "bar", p.OAuthProviders.SlackCredentials.ClientSecret)
 			},
 		},
 	}

lib/config/fileconf.go

@@ -1321,7 +1321,11 @@ func (p *PluginOAuthProviders) Parse() (servicecfg.PluginOAuthProviders, error)
 		if err != nil {
 			return out, trace.Wrap(err)
 		}
-		out.Slack = slack
+		out.Slack = &oauth2.ClientCredentials{
+			ID:     slack.ClientID,
+			Secret: slack.ClientSecret,
+		}
+		out.SlackCredentials = slack
 	}
 	return out, nil
 }
@@ -1335,7 +1339,7 @@ type OAuthClientCredentials struct {
 	ClientSecret string `yaml:"client_secret"`
 }
 
-func (o *OAuthClientCredentials) Parse() (*oauth2.ClientCredentials, error) {
+func (o *OAuthClientCredentials) Parse() (*servicecfg.OAuthClientCredentials, error) {
 	if o.ClientID == "" || o.ClientSecret == "" {
 		return nil, trace.BadParameter("both client_id and client_secret paths must be specified")
 	}
@@ -1354,9 +1358,9 @@ func (o *OAuthClientCredentials) Parse() (*oauth2.ClientCredentials, error) {
 	}
 	clientSecret = strings.TrimSpace(string(content))
 
-	return &oauth2.ClientCredentials{
-		ID:     clientID,
-		Secret: clientSecret,
+	return &servicecfg.OAuthClientCredentials{
+		ClientID:     clientID,
+		ClientSecret: clientSecret,
 	}, nil
 }
 

lib/service/servicecfg/auth.go

@@ -178,7 +178,18 @@ type HostedPluginsConfig struct {
 // PluginOAuthProviders holds application credentials for each
 // 3rd party API provider
 type PluginOAuthProviders struct {
+	// TODO(tross) delete once teleport.e has been converted.
+	// Deprecated: use SlackCredentials instead.
 	Slack *oauth2.ClientCredentials
+
+	SlackCredentials *OAuthClientCredentials
+}
+
+// OAuthClientCredentials stores the client_id and client_secret
+// of an OAuth application.
+type OAuthClientCredentials struct {
+	ClientID     string
+	ClientSecret string
 }
 
 // KeystoreConfig configures the auth keystore.