Use a URL object in OpenInAppResponse

[ishank011]

This PR has a breaking change. Previously, we returned a string as the URL as part of OpenInAppResponse. Now, we return a complete URL object which also specifies the method to be used to call the URL, the headers, and the form parameters to be specified. Related cs3org/reva#1923

cc @wkloucek

[wkloucek]

I read up on forms a little bit. The GET method on a form submit will add the form fields to the query parameters in the URL whereas the POST method will put them in the body (https://developer.mozilla.org/en-US/docs/Learn/Forms/Sending_and_retrieving_form_data).

But in neither case there is an explicit option to set Headers. How would it be used then?

[ishank011]

It is possible to pass headers to iframes, albeit not in a straightforward manner https://stackoverflow.com/a/60571237. The apps which we're considering don't need any to be set but it would be a good functionality to have IMO.

[ishank011]

I've added a couple of fields to ProviderInfo to store the icon URI and a desktop_only parameter based on this requirement. Now we return the complete provider object instead of just the name so the clients can use the icon field and we can filter these out in the HTTP layer based on the request's user-agent.

[glpatcern]

As discussed, I'm merging this given that we tested the whole chain and we are ready to merge cs3org/reva#1968 as well.