Implement endpoint for fetching available languages on org-level

[ErlingHauan]

Description

As a frontend developer, I need access to the languages that are listed in the text resource files in the {org}-content repo.

The new endpoint will be very similar to the "languages" endpoint found in TextController.cs on app-level, but since the folder structure is slightly different in the {org}-content repo, we need separate controller, service, and repository methods.

Acceptance criteria

• Create a GET endpoint in OrgTextController that returns languages as a list of strings.