Add a portal implementation

[A6GibKm]

There are a few things missing

• [ ] Use meson to build and install the binary together with the .portal and .service files
• [x] Find a better name
• [x] Clean code a bit
• [x] Is Request doing the correct thing?

This is compatible with gnome-keyring's stored secrets, but we store them with a more descriptive label, e.g.

Secret Portal token for com.belmoussaoui.ashpd.demo

See https://github.com/flatpak/xdg-desktop-portal/issues/482.

[bilelmoussaoui]

Use meson to build and install the binary together with the .portal and .service files

This is something we can do later imho

[A6GibKm]

From my point of view, the only thing missing here is more testing.

[bilelmoussaoui]

We can have more testing once we get the server bits re-implemented. Then we can write a mock implementation and have all the bits tested. For now, let us land it