[K2] Support legacy serialization for custom function type extensions.

For Compose libraries, it is important that libraries built with
K2 in version 2.0 can be read and used by a version 1.9 K1
compiler. That is not possible if custom function types are
in the metadata.

^KT-58456 Fixed

generators/tests/org/jetbrains/kotlin/generators/tests/GenerateTests.kt

@@ -19,6 +19,7 @@ import org.jetbrains.kotlin.assignment.plugin.AbstractFirLightTreeBlackBoxCodege
 import org.jetbrains.kotlin.assignment.plugin.AbstractFirPsiAssignmentPluginDiagnosticTest
 import org.jetbrains.kotlin.assignment.plugin.AbstractIrBlackBoxCodegenTestAssignmentPlugin
 import org.jetbrains.kotlin.fir.plugin.runners.AbstractFirLightTreePluginBlackBoxCodegenTest
+import org.jetbrains.kotlin.fir.plugin.runners.AbstractFirLoadK2CompiledKotlinWithPluginTest
 import org.jetbrains.kotlin.fir.plugin.runners.AbstractFirPsiPluginDiagnosticTest
 import org.jetbrains.kotlin.generators.generateTestGroupSuiteWithJUnit5
 import org.jetbrains.kotlin.generators.impl.generateTestGroupSuite
@@ -262,6 +263,10 @@ fun main(args: Array<String>) {
             testClass<AbstractFirLightTreePluginBlackBoxCodegenTest> {
                 model("box")
             }
+
+            testClass<AbstractFirLoadK2CompiledKotlinWithPluginTest> {
+                model("firLoadK2Compiled")
+            }
         }
 
         testGroup(