message: Add DBusVariant, a way to copy a single message item

message: Add DBusVariant, a way to copy a single message item

For #100344, we will need a way to store the metadata from the
original method call, and copy them back into arbitrarily many
messages later. This would be easy in GDBus, which has GVariant
as a first-class object. However, libdbus doesn't have an object for
message items, only messages.

We could copy the message's content, but it will carry file descriptors,
which we don't want to copy. Instead, introduce an internal object
representing a message item in a small buffer. It is stored as a variant
(D-Bus type 'v') so that it naturally carries its own type.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=101568