Add plugin for nextcloud-login

Test Plan:
1. Apply this patch and build the project.
2. Build plasma-settings project to test the plugin.
3. Open Accounts item. Tap `Add Account` button.
4. Select nexcloud from options for nextcloud-login.

Reviewers: bshah, jbbgameich, rpatwal

Reviewed By: bshah

Subscribers: nicolasfella

Differential Revision: https://phabricator.kde.org/D21738