Add prompt.auto-init buildstream.conf option

Provide an option in buildstream.conf to disable the 'Would you like to
...' prompt when we cannot resolve a project.

Some users prefer not to be interrupted by such prompts, so pave the way
to creating options to disable all those that might get in the way.

Follow the example of the advice.* options 'git-config', and create a
namespace for these UI options grouped by behaviour, rather than an
over-reaching 'ui.*' namespace. In later work perhaps we'll also add
'advice.*' options.

Add a NEWS item for this.