fix scheduling posts (#4392)

Mastodon returns different reponses when posting normally and when
scheduling. This was previously ignored silently, but Moshi is more
correct than Gson and fails, which causes the `SendStatusService` to
retry sending forever and a lot of posts are scheduled.
Mastodon should actually ignore multiple attempts at scheduling the same
post, but doesn't so I filed this
https://github.com/mastodon/mastodon/issues/30039

cc @cbeyls

app/src/main/java/com/keylesspalace/tusky/network/MastodonApi.kt

@@ -199,6 +199,14 @@ interface MastodonApi {
         @Body status: NewStatus
     ): NetworkResult<Status>
 
+    @POST("api/v1/statuses")
+    suspend fun createScheduledStatus(
+        @Header("Authorization") auth: String,
+        @Header(DOMAIN_HEADER) domain: String,
+        @Header("Idempotency-Key") idempotencyKey: String,
+        @Body status: NewStatus
+    ): NetworkResult<ScheduledStatus>
+
     @GET("api/v1/statuses/{id}")
     suspend fun status(@Path("id") statusId: String): NetworkResult<Status>