CUST-4527 added 'unknown' as a conferencing provider (#287)

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Author: samuelpx

src/main/kotlin/com/nylas/models/ConferencingProvider.kt

@@ -23,4 +23,7 @@ enum class ConferencingProvider {
 
   @Json(name = "skypeForConsumer")
   SKYPE_FOR_CONSUMER,
+
+  @Json(name = "unknown")
+  UNKNOWN,
 }