Proxy: Forward requests as partition and use it in standalone

The idea is to make it possible while embedding SlapOS in an existing partition to configure that some requests are forwarded to the master of the partition and to make these requests as the partition - so that when this partition is destroyed all other partitions from the embedded partitions are destroyed.

A bit more integration in slapos is still needed, but is intended to be used in theia (and why not webrunner):

At the beginning theia's instance  looks like this:

![screenshot of theia hosting subscription on slapos master, with the default 3 partitions](/uploads/9a297dbbae49996fb7ee69e89ea75da1/image.png)

after an ERP5 instance is requested in theia's embedded slapos, the partition requests are forwarded as this partition and appear here, as requested by theia:

![screenshot of theia hosting subscription on slapos master, frontend partitions requested by ERP5 appears as requested by theia](/uploads/37b8b30ca117a1251fc2bd099a5dac82/image.png)

This can also allow transparent integration, because the partition credentials are known by the partition, unlike the user credentials.

See merge request !218