Allow plugins to store dashboards in a custom folder

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: A plugin can currently include dashboards which the user can import into their Grafana instance https://grafana.com/developers/plugin-tools/reference-plugin-json#includes These dashboards are _always_ imported into the `General` folder. This can present a challenge if the dashboards should only be viewable by a subset of users, and more broadly can make it harder to discern where the dashboards have come from - i.e. making it obvious to users where Grafana apps have contributed these. **What would you like to be added**: Ability for plugins to specify a folder in which dashboards could be stored. We will need to explore the RBAC considerations for this, for example to ensure users who can access the app/plugin can access the folder and the inverse to also be true. A potential alternative would be to create a folder using the plugin name, by default set the permissions the same as general, but allow for this to be changed by administrators. **Who is this feature for?** Plugin authors, Grafana administrators **Workaround(s)** - It is possible for a plugin to bypass the standard include mechanism and instead including in the plugin package, as per synthetic monitoring.