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.