fix: remove serverUrl as default value for configuration (#34)

* fix: remove US URL as default

* fix: Make serverUrl init param as optional and nil by default

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Co-authored-by: Mykola Vaniurskyi <nicvan@ukr.net>

Author: kolyan94

Sources/Amplitude/Configuration.swift

@@ -46,7 +46,7 @@ public class Configuration {
         flushMaxRetries: Int = Constants.Configuration.FLUSH_MAX_RETRIES,
         useBatch: Bool = false,
         serverZone: ServerZone = ServerZone.US,
-        serverUrl: String = Constants.DEFAULT_API_HOST,
+        serverUrl: String? = nil,
         plan: Plan? = nil,
         ingestionMetadata: IngestionMetadata? = nil,
         trackingOptions: TrackingOptions = TrackingOptions(),