Support pcloud storage backend

[dtantono]

Describe the bug
I tried to connect the webdav for Authpass client in iOS. But, it does not work on PCloud with 2fa activated

To Reproduce
Steps to reproduce the behavior:

1. Go to Authpass
2. Click on Yes, open my password
3. Scroll down to Load from webdav
4. See error

Expected behavior
it should work on PCloud with 2fa

Screenshots

Smartphone (please complete the following information):

• Device: iPhone SE2020
• OS: iOS 15
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.

[hpoul]

are you sure pcloud supports 2fa for webdav? https://www.reddit.com/r/pcloud/comments/eah56g/2fa_and_webdav/

[hpoul]

(I don't even know how webdav is supposed to work with 2fa.. doesn't really make sense to me..)

[dtantono]

or you can use their API https://docs.pcloud.com/ because I have other app like keepass2android that support PCloud protocol and they can communicate with Pcloud account with 2fa

[hpoul]

Not really a priority for me right now.. but I'd hallily review PRs..
Adding support for another cloud storage backend is pretty straight forward, just needs to implement CloudStorageProvider:

authpass/authpass/lib/cloud_storage/cloud_storage_provider.dart

Line 153 in 81b0a73

abstract class CloudStorageProvider {