Add API to control whether polkit authorisation checks allow interaction

[jhenstridge]

This branch is intended to complement snapcore/snapd#3795 by adding set_allow_interaction and get_allow_interaction APIs to SnapdClient. The idea is to let the client decide whether snapd's polkit authorisation checks should allow interaction or not.

One question is what the default should be for snapd-glib. At present I've set it to default to allow interaction on the assumption that most users of the library will be graphical. But it would be pretty easy to disallow interaction by default.

The change should be backward compatible with old versions of snapd: they will simply ignore the extra request header.

[robert-ancell]

Looks good to me, happy to merge once snapd approves the use of this field.

[robert-ancell]

I brought over some of your test case changes into master, so this branch will have to be rebased again.

[jhenstridge]

I've updated the branch, also changing the value we set the request header to, as requested in the snapd PR.

[robert-ancell]

I squashed the commits and fixed up some documentation issues in master for this, thanks!