add option to change download_tasks.sql path to NSLibraryDirectory on ios

### Problem

Currently the download_tasks.sql file is stored in the documents directory on iOS. When one uses [UIFileSharingEnabled ](https://medium.com/@sasude9/download-files-in-files-app-in-ios-11-b676b89cf6be) for example to make downloaded files available in the Files app, download_tasks.sql also show up in that directory. According to [Apple](https://developer.apple.com/library/archive/documentation/FileManagement/Conceptual/FileSystemProgrammingGuide/FileSystemOverview/FileSystemOverview.html) one shouldn't store none-user files in the DocumentDirectory. Instead the NSLibraryDirectory is recommended. Moreover the flutter `path_provider` package suggest to use `getLibraryDirectory` for database files.


### Solution

I suggest to give users the option to specify where they want to store the database file.