Planning for flatpak 1.10

[Alexander Larsson]

Last week I did I started doing some planning for Flatpak 1.10, with
the goal of doing a release around the end of the year.

Feature-wise I think we're in a pretty good state, so there will not be
a lot of feature work. Instead, the main work will be on repository
scalability. Right now, the repository format is based on the summary
file, which has to be downloaded in its entirety before doing
anything. 

The summary contains information about all the refs in the repository
including the app metadata (permissions, dependencies, etc) for each
app. Additionally it contains all the delta files available in the
repo. To make matters worse, all this info is duplicated for each
supported architecture, making adding new arches very expensive.

So, some possible options we have here are:
 * Some form of delta for summaries
 * Split summaries in per-arch
 * Move delta info out of the summary file
 * Use more efficient summary format

Additionally, while we're redoing the summary and repo format we want
to make sure to:
 * Repository subseting (i.e. FSF free subset of flathub)
 * Improve the download size estimates (for the non-delta case)

I also want to look at some minor features:

Explore the possibility of using cgroups v2 for per-app limits (cpu,
memory, etc).

Explore the possibility of using slirp4netns to do network filtering.

Support debuginfod in flatpak/flathub to allow automatic on-demand
debug info download. However, it is possible we can do this entirely
without support from flatpak.

I also want need to make a pass over the terminal output/redraw
handling in flatpak, as it needs some polish.