Add support for `LVFS::UpdateMessage`
In https://github.com/fwupd/fwupd/issues/1383 there was some discussion that frontends will need to adopt support for LVFS::UpdateMessage
to be able to show messages after completing an update if they're present in the CAB.
This was implemented in the text frontends fwupdmgr
/fwupdtool
, but needs to be brought into other frontends now. Sample code from fwupdmgr
/fwupdtool
implementation is here: https://github.com/fwupd/fwupd/commit/32241f4256bfbc6d1042e7ae725edb7d7906dc59