[feat]: add default value for env ref backend

[MatteoVoges]

Description of the feature

On nexenio/dev I already implemented some features regarding env ref backend. With that you don't have to store a reffile, which has the advantage to just fetch the env value and don't need a reffile. This is important, because of the incompability with embed_refs, which is neccessary for file-less reveal processes using argocd.

Now the problem is, that there is no option for defining a default value, if the env variable could not get found.

Additional context

No response