Setup Caddy Webserver for testing/CI

Description

PR #10669 introduced the Caddy Webserver to be able to run the few test cases we have for HTTP/2 Server push. This is used in CI for several jobs, but not for Windows, so I thought I'll have a look. Since I found no further documentation in the code base, I looked at the implementation, and found:

https://github.com/php/php-src/blob/332e9a47ae66ac7db52a66e3bbff3469ef542d0c/.github/actions/setup-caddy/action.yml#L10-L12

Okay, on Windows we're using Chocolatey for some setup, so I checked whether they provide the Caddy Webserver, and found https://community.chocolatey.org/packages/caddy. Unfortunately, they apparently do not provide a way to use a custom Caddyfile, which is required for our test suite.

So I downloaded Caddy Webserver manually, and started it in an elevated shell. A message box popped up asking whether I want to install the local certificate into the trust store; clicked okay. Then I tried a request using curl, but that failed (CRYPT_E_NO_REVOCATION_CHECK). Took me a while to realize that the import into the trust store hadn't actually worked. So finally copy/pasted the local certificate into a cacert.pem file downloaded from the curl Website, and then pointing to this file (curl --cacert). That still did not work for my system curl, which uses Schannel, but fine with our own curl build.

To be able to run the tests, I've then added a respective curl.cainfo option to php.ini, and finally could run the tests.

It might not be worth to integrate this for our Windows CI jobs (after all, there are only 4 tests requiring Caddy Webserver, and HTTP/2 push might be obsolete), but maybe a few lines of documentation would make sense, so (Windows) developers who want/need to look into this don't need to figure out how to setup a test environment for these tests themselves. As I understand it, README files in the test directories are no longer desired; would some documentation in the docs/ folder be more appropriate?

Lies .github/actions/setup-caddy/action.yml und untersuche die vier Tests, die Caddy Webserver erfordern. Dokumentiere die im Issue beschriebenen Windows-Setup-Schritte einschließlich der benutzerdefinierten Caddyfile und der Zertifikatshandhabung am geeigneten Dokumentationsort; als abgeschlossen gilt die Aufgabe, wenn ein Windows-Entwickler die Testumgebung reproduzieren kann, ohne diese Untersuchung wiederholen zu müssen.