add simpleProxyFetcher and add fetcher unit tests

src/fetchers/simpleProxy.ts

@@ -0,0 +1,35 @@
+import fetch from 'node-fetch';
+
+import { makeFullUrl } from '@/fetchers/common';
+import { makeStandardFetcher } from '@/fetchers/standardFetch';
+import { Fetcher } from '@/fetchers/types';
+
+const headerMap: Record<string, string> = {
+  cookie: 'X-Cookie',
+  referer: 'X-Referer',
+  origin: 'X-Origin',
+};
+
+export function makeSimpleProxyFetcher(proxyUrl: string, f: typeof fetch): Fetcher {
+  const fetcher = makeStandardFetcher(f);
+  const proxiedFetch: Fetcher = async (url, ops) => {
+    const fullUrl = makeFullUrl(url, ops);
+
+    const headerEntries = Object.entries(ops.headers).map((entry) => {
+      const key = entry[0].toLowerCase();
+      if (headerMap[key]) return [headerMap[key], entry[1]];
+      return entry;
+    });
+
+    return fetcher(proxyUrl, {
+      ...ops,
+      query: {
+        destination: fullUrl,
+      },
+      headers: Object.fromEntries(headerEntries),
+      baseUrl: undefined,
+    });
+  };
+
+  return proxiedFetch;
+}