GEOS-11555: fix installation of extensions in various configurations

[spike83]

With this PR, I want to address the issues GEOS-11555 and GEOS-11258 about some inabilities of the extension installer script.

In my opinion, it should be possible to add community modules also to official images. So, I added a warning message instead of blocking it.

Also, I tried to get rid of the confusion in the naming of the variable INSTALL_EXTENSIONS which was controlling the download behavior. Thus, I introduced a variable DOWNLOAD_EXTENSIONS which controls this and made the INSTALL_EXTENSIONS to really control the installation of extensions.

On one point, I'm not sure: Should we fail the startup when we are unable to download extensions? On one hand, the full functionality might not he available and cannot be detected with any sort of health check and on the other hand, maybe it's better to run without an extension than not run at all.