Add `Apache httpd` dynamic application build from source

[StefanJum]

Add the Apache httpd applications as a dynamic build, from the application source files.
Test it using app-elfloader.
If any problems appear, open issues on the Unikraft core repository.

The typical process of adding apps to the dynamic-apps repository, built from the source files, is:

• Create the app directory that will store: the executable, depending shared libraries, configuration files, data files, etc. Name it app-name-source (e.g. app-redis-source).
• Download the source code for the application (use apt source or find the source code somwhere else, like GitHub)
• Build the application following the instructions provided.
• Update or create any configuration files, if required.
• Run the executable locally, to test it.
• Run the extract.sh script to pack the application.
• Build a bash script that does all the steps you just did (downloads the sources, builds the application, creates the config files, calls the extract.sh script).
  You can take a peek at the static-pie scripts (app-nginx for example).
• Now profit. Use app-elfloader.

Hackathon points: 40