Add option to store key on user device when end-to-end encryption is enabled

[tobiasKaminsky]

See https://github.com/nextcloud-gmbh/customer-feature-requests/issues/29

[mgallien]

I switch to not using openssl directly but via libp11 that is going to make connecting to the hardware driver much quicker

[mgallien]

Driver path could come up at installation time via MSI parameters or brander for other platforms.

[mgallien]

Basic end-to-end encryption with hardware token is now working
I assume I spent already 3 days out of the 5 expected days
I assume that will not be enough to complete the work

[mgallien]

windows is now basically working (with a few workarounds including to compile and deploy libp11 PKCS#11 library we now uses)

[mgallien]

still need to adjust stuff but windows installer (of not final state) is almost there
still having build script in unrelated repository makes it complicated to test (see https://github.com/nextcloud/client-building/)