[Feature] Export and import ObjectBox database as a `.mdb` file from the app

[shubham0204]

ObjectBox uses a single .mdb file to save data, and has APIs to load the data from a file. If the user has a large number of documents, it will be convenient to have an option to export the entire database (chunks + docs) as a single file and load them in another instance of the app.

• ObjectBox FAQ: Can I ship my app with a pre-built database?