support SwissTopo STAC API

[cholmes]

Trying to add https://data.geo.admin.ch/api/stac/v0.9/ I get 'Connection Failed; http error':

It looks to be a 0.9.0 STAC API/core, not sure what the backend is.

[volaya]

If the url is added without the trailing slash, it works fine.

I made a small PR to automalically clean the URL of any trailing slash that it might have:

#87

However, after that, it shows the collection, but the search does not work. There are several issues:

1. the server expects the datetime property (as recommended in the STAC API docs) instead of time (as the plugin does). This is fixed already in a PR that is not yet merged:

#79

2. Even with that, the server does not process correctly the request and the search fails. It seems to be a server issue. I tried manually outside of the plugin, with well-formed searches that work on other services, and it keeps failing.