common/kafka: add support for OAuth2

The support is still pretty basic. Notably, scopes are not
configurable (waiting for someone to request them) and maybe there
client ID and secrets should not be provided as username/password.

Fix #1714

common/kafka/config_test.go

@@ -62,6 +62,19 @@ func TestKafkaNewConfig(t *testing.T) {
 					Mechanism: SASLScramSHA512,
 				},
 			},
+		}, {
+			description: "SASL OAuth2",
+			config: Configuration{
+				TLS: helpers.TLSConfiguration{
+					Enable: true,
+				},
+				SASL: SASLConfiguration{
+					Username:      "hello",
+					Password:      "password",
+					Mechanism:     SASLOauth,
+					OAuthTokenURL: "http://example.com/token",
+				},
+			},
 		},
 	}
 	for _, tc := range cases {
@@ -187,6 +200,65 @@ func TestTLSConfiguration(t *testing.T) {
 					Mechanism: SASLScramSHA256,
 				},
 			},
+		}, {
+			Description: "TLS SASL OAuth",
+			Initial:     func() interface{} { return DefaultConfiguration() },
+			Configuration: func() interface{} {
+				return gin.H{
+					"tls": gin.H{
+						"enable": true,
+					},
+					"sasl": gin.H{
+						"username":        "hello",
+						"password":        "bye",
+						"mechanism":       "oauth",
+						"oauth-token-url": "http://example.com/token",
+					},
+				}
+			},
+			Expected: Configuration{
+				Topic:   "flows",
+				Brokers: []string{"127.0.0.1:9092"},
+				Version: Version(sarama.V2_8_1_0),
+				TLS: helpers.TLSConfiguration{
+					Enable: true,
+					// Value from DefaultConfig is true
+					Verify: true,
+				},
+				SASL: SASLConfiguration{
+					Username:      "hello",
+					Password:      "bye",
+					Mechanism:     SASLOauth,
+					OAuthTokenURL: "http://example.com/token",
+				},
+			},
+		}, {
+			Description: "OAuth requires a token URL",
+			Initial:     func() interface{} { return DefaultConfiguration() },
+			Configuration: func() interface{} {
+				return gin.H{
+					"sasl": gin.H{
+						"username":  "hello",
+						"password":  "bye",
+						"mechanism": "oauth",
+					},
+				}
+			},
+			Error: true,
+		}, {
+			Description: "OAuth token URL only with OAuth",
+			Initial:     func() interface{} { return DefaultConfiguration() },
+			Configuration: func() interface{} {
+				return gin.H{
+					"sasl": gin.H{
+						"username":        "hello",
+						"password":        "bye",
+						"mechanism":       "plain",
+						"oauth-token-url": "http://example.com/token",
+					},
+				}
+			},
+			Error: true,
 		},
 	})
 }