Allow access to (eg. WiFi) secrets

[chrysn]

Proposed Changes

This adds a function that wraps the GetSecrets method.

Draft status

It is a draft because I'm still unsure on two aspects:

• How much of the conventional API inside the stable DBus API should be covered? Should the input be an enum (probably yes), or should there be a function per kind of secret (don't know), and if so, should the library process the {s: v} into a Rust struct (probably not, assuming that this is where faster-than-API extension in NetworkManager happens)?
• Should the DBus-level NetworkManager Connection (as opposed to the ActiveConnection that is represented by the current Connection struct) be exposed as an explicit Thing?
  This would eliminate what is currently a tiny race condition if the network is changed while the SSID/password is being queried: for then, there might be the SSID of the one and the secrets of the other being returned.