x/sdk: schema_registry_example

Depends on avsc, which has some weird polyfill (?) issues w/ finding
buffer.SlowBuffer. We also need to address the issue of wasm-merging
a whole node project in the GHA container. But with some massaging,
this example does pass the integration test suite for transforms.

Signed-off-by: Oren Leiman <[email protected]>

src/transform-sdk/js/schema_registry_example/.gitignore

@@ -0,0 +1,3 @@
+node_modules
+node_modules/**/*
+user_code.js

src/transform-sdk/js/schema_registry_example/esbuild.js

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+import * as esbuild from 'esbuild'
+import { polyfillNode } from "esbuild-plugin-polyfill-node";
+
+await esbuild.build({
+  entryPoints: ["src/index.ts"],
+  outfile: "dist/schema_registry_example.js",
+  bundle: true,
+  external: [
+    // This package is provided by the Redpanda JavaScript runtime.
+    "@redpanda-data/transform-sdk",
+    "@redpanda-data/transform-sdk-sr",
+  ],
+  target: "es2022",
+  platform: "neutral", // We're running in Wasm
+  plugins: [
+    polyfillNode({
+      globals: {
+        // Allow a global Buffer variable if referenced.
+        buffer: true,
+        // Don't inject the process global, the Redpanda JavaScript runtime
+        // does that.
+        process: false,
+      },
+      polyfills: {
+        // Any NodeJS APIs that need to polyfilled can be added here.
+      },
+    }),
+  ],
+});