Add Mattermost as ConfigType for notifications channel (#853)

Signed-off-by: Craig Perkins <cwperx@amazon.com>

Author: cwperks

src/main/kotlin/org/opensearch/commons/notifications/model/ConfigType.kt

@@ -59,6 +59,11 @@ enum class ConfigType(val tag: String) {
         override fun toString(): String {
             return tag
         }
+    },
+    MATTERMOST("mattermost") {
+        override fun toString(): String {
+            return tag
+        }
     };
 
     companion object {

src/main/kotlin/org/opensearch/commons/notifications/model/config/ConfigDataProperties.kt

@@ -38,7 +38,8 @@ internal object ConfigDataProperties {
         Pair(ConfigType.SES_ACCOUNT, ConfigProperty(SesAccount.reader, SesAccount.xParser)),
         Pair(ConfigType.EMAIL_GROUP, ConfigProperty(EmailGroup.reader, EmailGroup.xParser)),
         Pair(ConfigType.SMTP_ACCOUNT, ConfigProperty(SmtpAccount.reader, SmtpAccount.xParser)),
-        Pair(ConfigType.MICROSOFT_TEAMS, ConfigProperty(MicrosoftTeams.reader, MicrosoftTeams.xParser))
+        Pair(ConfigType.MICROSOFT_TEAMS, ConfigProperty(MicrosoftTeams.reader, MicrosoftTeams.xParser)),
+        Pair(ConfigType.MATTERMOST, ConfigProperty(Slack.reader, Slack.xParser))
     )
 
     /**
@@ -65,6 +66,7 @@ internal object ConfigDataProperties {
             ConfigType.SNS -> configData is Sns
             ConfigType.SES_ACCOUNT -> configData is SesAccount
             ConfigType.MICROSOFT_TEAMS -> configData is MicrosoftTeams
+            ConfigType.MATTERMOST -> configData is Slack
             ConfigType.NONE -> true
         }
     }