Support steam:// links

**Game information**

Not really, just the integration between OS and Steam flatpak

**Distribution name and version where applicable**
```
NAME="Fedora Linux"
VERSION="36.20220707.0 (Silverblue)"
```

**Flatpak info**

```
[mateusrc@centauro ~]$ flatpak --version
Flatpak 1.12.7
[mateusrc@centauro ~]$ flatpak --gl-drivers
nvidia-515-57
default
host
```

**Problem description**

It seem the Steam flatpak doesn't register as a handler for `steam://` links.
This prevents external programs from asking Steam to install or launch games.

**Does this issue reproduce with native Steam**

Have not tested