Make setting `CPL_VSIL_CURL_ALLOWED_EXTENSIONS`mandatory

On some projects, this variable is necessary for performance reasons, on others it's critical that it's not set.

So we should not provide a default but force it to be set in some way.

It could be a helm value such as:

global:
    fileExtensionsFilter:
        enabled: true        
        values: ".tif,.xml"

enabled would be e.g. null by default which should raise an error.

Internally, this could set the respective env var, or we implement a more specific mechanism in eoxserver, which would allow us to also use different settings for different use cases. This would however complicate the config here.