BE: GET endpoint for generating qr codes

[jdmchugh111]

Using the rqr_code gem

This would generate a qr code if it did not exist, otherwise it returns the existing qr code from the database

Would need to run a migration to add a qr_code attribute to links and set the default to null/nil.

A user would have the option to generate a QR code for any of their links. This would then send the GET request to the backend and a qr code would be generated and stored as an attribute in the link