chore: able to get all translated strings, filter them by group, language and search by string

[simptheshrimp]

We need to be able to get all translated strings to our Gatsby frontend, per language and per group. We introduced a new JSON scalar which we have tested live for the past 4 months in about 70 sites and has been working without issues.

[esamattis]

I'm sorry but I don't think I'm able to merge this without tests. If there's no tests there's no way we can ensure this feature will work with in future and just causes us extra maintenance burden when it breaks.