lib/remote: Add getter/setter for collection IDs

This isn’t really used internally, but will be used by gnome-software
for when it configures new flatpak remotes.

This is new public API, but is only declared if compiling with
--enable-p2p.

Includes some basic smoketests.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
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