[FIR] Make FirExtensionRegistrar a proper extension point

After that commit it's allowed to register any FIR plugin using
  existing extension infrastructure (`-Xplugin` CLI argument,
  plugin.xml for IDE plugins, etc)

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/extensions/FirExtensionRegistrar.kt

@@ -5,14 +5,18 @@
 
 package org.jetbrains.kotlin.fir.extensions
 
+import org.jetbrains.kotlin.extensions.ProjectExtensionDescriptor
 import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.SessionConfiguration
 import org.jetbrains.kotlin.fir.analysis.extensions.FirAdditionalCheckersExtension
 import kotlin.reflect.KClass
 
 abstract class FirExtensionRegistrar {
-    companion object {
-        val AVAILABLE_EXTENSIONS = listOf(
+    companion object : ProjectExtensionDescriptor<FirExtensionRegistrar>(
+        name = "org.jetbrains.kotlin.fir.extensions.firExtensionRegistrar",
+        extensionClass = FirExtensionRegistrar::class.java
+    ) {
+        internal val AVAILABLE_EXTENSIONS = listOf(
             FirStatusTransformerExtension::class,
             FirDeclarationGenerationExtension::class,
             FirAdditionalCheckersExtension::class,

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/session/FirSessionFactory.kt

@@ -84,6 +84,7 @@ object FirSessionFactory {
         librariesScope: AbstractProjectFileSearchScope,
         lookupTracker: LookupTracker?,
         providerAndScopeForIncrementalCompilation: ProviderAndScopeForIncrementalCompilation?,
+        extensionRegistrars: List<FirExtensionRegistrar>,
         dependenciesConfigurator: DependencyListForCliModule.Builder.() -> Unit = {},
         noinline sessionConfigurator: FirSessionConfigurator.() -> Unit = {},
     ): FirSession {
@@ -113,6 +114,7 @@ object FirSessionFactory {
             sourceScope,
             projectEnvironment,
             providerAndScopeForIncrementalCompilation,
+            extensionRegistrars,
             languageVersionSettings = languageVersionSettings,
             lookupTracker = lookupTracker,
             init = sessionConfigurator
@@ -125,6 +127,7 @@ object FirSessionFactory {
         scope: AbstractProjectFileSearchScope,
         projectEnvironment: AbstractProjectEnvironment,
         providerAndScopeForIncrementalCompilation: ProviderAndScopeForIncrementalCompilation?,
+        extensionRegistrars: List<FirExtensionRegistrar>,
         languageVersionSettings: LanguageVersionSettings = LanguageVersionSettingsImpl.DEFAULT,
         lookupTracker: LookupTracker? = null,
         init: FirSessionConfigurator.() -> Unit = {}
@@ -160,6 +163,9 @@ object FirSessionFactory {
             FirSessionConfigurator(this).apply {
                 registerCommonCheckers()
                 registerJvmCheckers()
+                for (extensionRegistrar in extensionRegistrars) {
+                    registerExtensions(extensionRegistrar.configure())
+                }
                 init()
             }.configure()