Nextcloud should default to autostart in libcloudprovider enabled environments.

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### Feature description
Nextcloud should default to auto-start (`--background`) itself on session start on systems who actively used libcloudprovider implementations.

#### Motivation
Currently Nextcloud is set to only sync after you start the client yourself. auto-start is disabled by default. I think this is a sane standard on normal systems, where you want to deal with the client and have actual controls in your tray, and the client later on.
On systems with libcloudproviders implementation it's different. Here, Nextcloud is handled as kind of session service, that only provides the functionality, but not the end user interface to deal with syncs. This is up to the user-facing part of the libcloudproviders implementation. Because of this there is no real reason for the user to launch the client from a menu.

#### Current Problems in the implementation
If not set to auto-start itself, Nextcloud is 'not' up and working until you launch it yourself or via file manager that is unexpected for a system service. (also compared to the likes of iCloud and oneDrive on macOS and Windows)

If Nextcloud is not set to auto-start, it might come up to late via DBus-activation and you will not see your shares in the file manager (see: [libcloudproviders-apps do not connect to Nextcloud-client on fast systems](https://gitlab.gnome.org/World/libcloudproviders/-/issues/19)). So you are unable to control your shares and might think that it isn't even running, while its up.

#### Implementation
In order to have this as easy as possible. The client should check if its actually build with libcloudproviders support and if libcloudproviders is available on the system at runtime. If both of them are true, Nextcloud should activate auto-start on client setup by default. Of course the option to disable auto-start should stay, in case the user does want to disable this and only use DBus activation or something else.
If false, nextcloud should act as standalone-app and keep the current behavior (disabled autostart by default).

#### Drawbacks
There could be a conflict on Linux-distros who ship libcloudproviders by default, but don't provide a user-facing application to control it.
Another one could be for users who don't want to auto-start Nextcloud, even on libcloudprovider-enabled systems.

Both of them are limited drawbacks that can be resolved by disabling the auto-start in the client