Actions: Pull and run httpbin as a service container

[ryonakano]

Changes Summary

• Run httpbin as a service container instead of using the officially deployed one in CI so that the build and test won't fail when the official one is down
  • Add a build option to set the HTTP server used in the integration tests

FYI: https://docs.github.com/en/actions/tutorials/use-containerized-services/use-docker-service-containers

Checklist

• The CI passes
• Local test using the default https://httpbin.org/post stil works (meson setup builddir --prefix=/usr -Dintegration_tests=true && ninja -C builddir/ test)

[ryonakano]

Closing as the officail httpbin works now.