Add custom codegen hook before shape generation

Some languages need to generate code before shapes are generated.
Previously such languages could have used the service generation hook
to perform this, but that was moved to occur after generating the
other shapes.

Author: JordonPhillips

smithy-codegen-core/src/main/java/software/amazon/smithy/codegen/core/directed/CodegenDirector.java

@@ -292,22 +292,26 @@ public void run() {
 
         LOGGER.fine("All setup done. Beginning code generation");
 
+        LOGGER.finest(() -> "Performing custom codegen for "
+                + directedCodegen.getClass().getName() + " before shape codegen");
+        CustomizeDirective<C, S> customizeDirective = new CustomizeDirective<>(context, serviceShape);
+        directedCodegen.customizeBeforeShapeGeneration(customizeDirective);
+
+        LOGGER.finest(() -> "Generating shapes for service " + serviceShape.getId());
         generateShapesInService(context, serviceShape);
 
         LOGGER.finest(() -> "Generating service " + serviceShape.getId());
         directedCodegen.generateService(new GenerateServiceDirective<>(context, serviceShape));
 
-        CustomizeDirective<C, S> postProcess = new CustomizeDirective<>(context, serviceShape);
-
         LOGGER.finest(() -> "Performing custom codegen for "
                             + directedCodegen.getClass().getName() + " before integrations");
-        directedCodegen.customizeBeforeIntegrations(postProcess);
+        directedCodegen.customizeBeforeIntegrations(customizeDirective);
 
         applyIntegrationCustomizations(context, integrations);
 
         LOGGER.finest(() -> "Performing custom codegen for "
                             + directedCodegen.getClass().getName() + " after integrations");
-        directedCodegen.customizeAfterIntegrations(postProcess);
+        directedCodegen.customizeAfterIntegrations(customizeDirective);
 
         LOGGER.finest(() -> "Directed codegen finished for " + directedCodegen.getClass().getName());
 

smithy-codegen-core/src/main/java/software/amazon/smithy/codegen/core/directed/DirectedCodegen.java

@@ -106,6 +106,16 @@ default void generateResource(GenerateResourceDirective<C, S> directive) {
      */
     void generateIntEnumShape(GenerateIntEnumDirective<C, S> directive);
 
+    /**
+     * Performs any necessary code generation before all shapes are generated,
+     * using the created codegen context object.
+     *
+     * @param directive Directive to perform.
+     */
+    default void customizeBeforeShapeGeneration(CustomizeDirective<C, S> directive) {
+        // Does nothing by default.
+    }
+
     /**
      * Performs any necessary code generation after all shapes are generated,
      * using the created codegen context object before integrations perform