feat(common): add json user pref service

ui/common/src/main/groovy/com/github/thetric/iliasdownloader/ui/common/prefs/JsonUserPreferenceService.groovy

@@ -0,0 +1,34 @@
+package com.github.thetric.iliasdownloader.ui.common.prefs
+
+import groovy.json.JsonOutput
+import groovy.json.JsonSlurper
+
+import java.nio.charset.StandardCharsets
+import java.nio.file.Path
+
+final class JsonUserPreferenceService implements UserPreferenceService {
+    private final JsonSlurper jsonSlurper
+    final Path settingsFile
+
+    JsonUserPreferenceService(Path settingsPath) {
+        this.settingsFile = settingsPath
+        jsonSlurper = new JsonSlurper()
+    }
+
+    @Override
+    UserPreferences loadUserPreferences() throws IOException {
+        return settingsFile.withInputStream {
+            def a = new UserPreferences(jsonSlurper.parse(it) as Map)
+            return a
+        } as UserPreferences
+    }
+
+    @Override
+    void saveUserPreferences(final UserPreferences userPreferences) throws IOException {
+        settingsFile.withWriter(StandardCharsets.UTF_8.name(), {
+            def compressedJson = JsonOutput.toJson(userPreferences)
+            def prettyPrintedJson = JsonOutput.prettyPrint(compressedJson)
+            it.write("$prettyPrintedJson\n")
+        })
+    }
+}