Add support for url scheme zoom

Hi, which snap specifically does not work with these zoom links? Can you also post the output of `snap version` ?

Lastly, we have added better support for snapd to forward requests to xdg-open from inside a strict snap to xdg-desktop-portal on the host if that is available, which should solve this sort of issue generically, can you try refreshing to snapd snap candidate channel (which is version 2.45) and try again? Specifically run:

```
snap install snapd --beta || snap refresh snapd --beta
```

Hey there!
It's specifically Chromium.
```
➜ ~ snap version
snap 2.44.3+20.04
snapd 2.44.3+20.04
series 16
ubuntu 20.04
kernel 5.4.0-31-generic
```

I had to run `snap refresh snapd --beta`, but after doing that:
```
➜ ~ snap refresh snapd --beta
2020-05-26T09:57:07-04:00 INFO Waiting for restart...
snapd (beta) 2.45+git12.g48ab8c0 from Canonical✓ refreshed
```

It's still the same as before (opening a Zoom link in Chromium still doesn't open Zoom). Do I need to reinstall Chromium, or do something else?

I'm not sure what you're asking me to do with xdg-desktop-portal, so I might be misunderstanding some parts. How can I check if it's "available"?

zoommtg should be supported in 2.45, we didn't get mentions or patches for zoomus ? what's the difference

I'm not sure what causes the difference, but sometimes the link will be zoomus. I assume it has something to do with how the host shares the link, but I don't know what exactly.

I just looked at the code and we indeed only support "zoommtg" and not "zoomus". This is documented on the zoom API page: https://marketplace.zoom.us/docs/guides/guides/client-url-schemes

We should support "zoomus" and "zoomphonecall" for complete experience.

I've sent a pull request with those two schemes: https://github.com/snapcore/snapd/pull/8910