Upstream configuration improvement: (#149)

- now we can specify connector mode for better fine tuning of connection

src/test/kotlin/io/emeraldpay/dshackle/config/UpstreamsConfigTest.kt

@@ -0,0 +1,61 @@
+package io.emeraldpay.dshackle.config
+
+import io.emeraldpay.dshackle.upstream.ethereum.connectors.EthereumConnectorFactory.ConnectorMode
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.params.ParameterizedTest
+import org.junit.jupiter.params.provider.Arguments
+import org.junit.jupiter.params.provider.MethodSource
+import java.net.URI
+import java.util.stream.Stream
+
+internal class UpstreamsConfigTest {
+    companion object {
+        @JvmStatic
+        fun data(): Stream<Arguments> {
+            return Stream.of(
+                Arguments.of(
+                    UpstreamsConfig.EthereumConnection(),
+                    ConnectorMode.RPC_ONLY
+                ),
+                Arguments.of(
+                    UpstreamsConfig.EthereumConnection()
+                        .apply {
+                            ws = UpstreamsConfig.WsEndpoint(URI("ws://localhost:8546"))
+                        },
+                    ConnectorMode.WS_ONLY
+                ),
+                Arguments.of(
+                    UpstreamsConfig.EthereumConnection()
+                        .apply {
+                            preferHttp = true
+                            ws = UpstreamsConfig.WsEndpoint(URI("ws://localhost:8546"))
+                        },
+                    ConnectorMode.RPC_REQUESTS_WITH_MIXED_HEAD
+                ),
+                Arguments.of(
+                    UpstreamsConfig.EthereumConnection()
+                        .apply {
+                            connectorMode = "RPC_REQUESTS_WITH_WS_HEAD"
+                            ws = UpstreamsConfig.WsEndpoint(URI("ws://localhost:8546"))
+                        },
+                    ConnectorMode.RPC_REQUESTS_WITH_WS_HEAD
+                ),
+                Arguments.of(
+                    UpstreamsConfig.EthereumConnection()
+                        .apply {
+                            preferHttp = true
+                            connectorMode = "RPC_REQUESTS_WITH_WS_HEAD"
+                            ws = UpstreamsConfig.WsEndpoint(URI("ws://localhost:8546"))
+                        },
+                    ConnectorMode.RPC_REQUESTS_WITH_MIXED_HEAD
+                ),
+            )
+        }
+    }
+
+    @ParameterizedTest
+    @MethodSource("data")
+    fun testKeepForwarded(input: UpstreamsConfig.EthereumConnection, expected: ConnectorMode) {
+        assertEquals(expected, input.resolveMode())
+    }
+}