Add Kotlin plugin system (SDK, host, hooks, settings UI)

Implement exteraGram-style plugin architecture for Android and desktop:
- plugins/sdk: BasePlugin API, hook types, settings DSL
- plugins/host: PluginEngine, typed send hooks, FeatureGate, HookRegistry
- JVM ByteBuddy and Android Pine hook backends
- DevServer on port 42690 (JVM)
- Plugins settings screens and PluginUiHost bulletin overlay
- hello_world sample plugin packaged as .fcplugin
- ProGuard keepnames for ru.fromchat.** with shrinking enabled

Co-authored-by: denis0001-dev <denis0001.dev@ya.ru>

plugins/sdk/build.gradle.kts

@@ -0,0 +1,20 @@
+plugins {
+    alias(libs.plugins.kotlin.multiplatform)
+    alias(libs.plugins.kotlin.serialization)
+    alias(libs.plugins.kotlin.multiplatform.library)
+}
+
+kotlin {
+    jvm()
+    androidLibrary {
+        namespace = "ru.fromchat.plugins.sdk"
+        compileSdk = 37
+        minSdk = 24
+    }
+
+    sourceSets {
+        commonMain.dependencies {
+            implementation(libs.kotlinx.serialization.json)
+        }
+    }
+}