Pass secret as file path for security

[V02460]

The sliding sync proxy currently only takes its secret via the environment variable SYNCV3_SECRET. When used with systemd this is not considered secure:

Note that environment variables are not suitable for passing secrets (such as passwords, key material, …) to service processes. Environment variables set for a unit are exposed to unprivileged clients via D-Bus IPC, and generally not understood as being data that requires protection. Moreover, environment variables are propagated down the process tree, including across security boundaries (such as setuid/setgid executables), and hence might leak to processes that should not have access to the secret data.

From https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html#Environment

Please change the sliding sync proxy to accept the path to a secret file.

[pcolladosoto]

Hi @V02460! I'd be more than happy to take a stab at this. I could have something working towards the end of the week: does that sound good?

[pcolladosoto]

This will hopefully be introduced on PR #446!