feat(aio): add API endpoint for notifying about PR updates

This commit adds an API endpoint for notifying the preview server about PR
updates (`/pr-updated`). According to the update, the preview server can take
several actions. Currently, it will only check and (if necessary) update the
PR's preview visibility (but more actions could be supported in the future).
The API can be used with an automatic trigger (e.g. a GitHub webhook) to
instantly update a PR's preview visibility when it changes.

Fixes #16526

aio/aio-builds-setup/dockerbuild/scripts-js/lib/verify-setup/nginx.e2e.ts

@@ -317,6 +317,51 @@ describe(`nginx`, () => {
     });
 
 
+    describe(`${host}/pr-updated`, () => {
+      const url = `${scheme}://${host}/pr-updated`;
+
+
+      it('should disallow non-POST requests', done => {
+        Promise.all([
+          h.runCmd(`curl -iLX GET ${url}`).then(h.verifyResponse([405, 'Not Allowed'])),
+          h.runCmd(`curl -iLX PUT ${url}`).then(h.verifyResponse([405, 'Not Allowed'])),
+          h.runCmd(`curl -iLX PATCH ${url}`).then(h.verifyResponse([405, 'Not Allowed'])),
+          h.runCmd(`curl -iLX DELETE ${url}`).then(h.verifyResponse([405, 'Not Allowed'])),
+        ]).then(done);
+      });
+
+
+      it('should pass requests through to the upload server', done => {
+        const cmdPrefix = `curl -iLX POST --header "Content-Type: application/json"`;
+
+        const cmd1 = `${cmdPrefix} ${url}`;
+        const cmd2 = `${cmdPrefix} --data '{"number":${pr}}' ${url}`;
+        const cmd3 = `${cmdPrefix} --data '{"number":${pr},"action":"foo"}' ${url}`;
+
+        Promise.all([
+          h.runCmd(cmd1).then(h.verifyResponse(400, /Missing or empty 'number' field/)),
+          h.runCmd(cmd2).then(h.verifyResponse(200)),
+          h.runCmd(cmd3).then(h.verifyResponse(200)),
+        ]).then(done);
+      });
+
+
+      it('should respond with 404 for unknown paths', done => {
+        const cmdPrefix = `curl -iLX POST ${scheme}://${host}`;
+
+        Promise.all([
+          h.runCmd(`${cmdPrefix}/foo/pr-updated`).then(h.verifyResponse(404)),
+          h.runCmd(`${cmdPrefix}/foo-pr-updated`).then(h.verifyResponse(404)),
+          h.runCmd(`${cmdPrefix}/foonpr-updated`).then(h.verifyResponse(404)),
+          h.runCmd(`${cmdPrefix}/pr-updated/foo`).then(h.verifyResponse(404)),
+          h.runCmd(`${cmdPrefix}/pr-updated-foo`).then(h.verifyResponse(404)),
+          h.runCmd(`${cmdPrefix}/pr-updatednfoo`).then(h.verifyResponse(404)),
+        ]).then(done);
+      });
+
+    });
+
+
     describe(`${host}/*`, () => {
 
       it('should respond with 404 for unknown URLs (even if the resource exists)', done => {