[FR] Ability to export from AppFlowy Cloud Beta and remove existing data

### Description

First of, thank you for AppFlowy. It is the first and only application I can use to track my personal tasks that fits my workflow instead of needing to learn a new one.

I was recently looking at the setting and noticed my cloud settings were set to "AppFlowy Cloud Beta". While this is a great feature and useful to many, I prefer my data to be local. I am not logged into any Oauth account, this is a local user (or so I think). I changed the cloud setting to be local and found the information from about October 2023. I switched back to "AppFlowy Cloud Beta" to find my current tasks. My apologies if I have missed this in the documentation, github issues and discussions, but I cannot seem to be able to find where there is the functionality, or proposed functionality, to backup/export from one cloud instance, whether it be AppFlowy's Cloud or a self hosted one, and import it to be used locally or migrate from AppFlowy's Cloud to a self hosted one. Additionally, users should be able to remove data stored on AppFlow's Cloud.

### Impact

* The ability to migrate data will allow users more freedom over where their information is stored.
* Removing unwanted cloud backups will save AppFlowy the storage overhead

### Additional Context

_No response_