Refactor Kitsu API to remove Retrofit usage

app/src/main/java/eu/kanade/tachiyomi/network/OkHttpExtensions.kt

@@ -5,6 +5,7 @@ import kotlinx.serialization.decodeFromString
 import kotlinx.serialization.json.Json
 import okhttp3.Call
 import okhttp3.Callback
+import okhttp3.MediaType.Companion.toMediaType
 import okhttp3.OkHttpClient
 import okhttp3.Request
 import okhttp3.Response
@@ -19,6 +20,8 @@ import java.util.concurrent.atomic.AtomicBoolean
 import kotlin.coroutines.resume
 import kotlin.coroutines.resumeWithException
 
+val jsonMime = "application/json; charset=utf-8".toMediaType()
+
 fun Call.asObservable(): Observable<Response> {
     return Observable.unsafeCreate { subscriber ->
         // Since Call is a one-shot type, clone it for each new subscriber.