feat(backend): add user can login with correct credentials SCRUM-52

[markkovari]

Based on @AnnaUgrai 's provided code.

Annotated the classes with documentation and sources.

Some of the methods were deprecated so I changed them to the new. Some todo's were added too, but those will be removed when we will have authenticated endpoints.

• write tests
• Add dependency

io.jsonwebtoken jjwt 0.12.6

• create login DTO
• create JWT utility class
• autowire it to the controller so we can add it to the login endpoint once it is created
  -> added it into the service rather
• configure spring security -> NOT YET NEEDED
• create jwt filter -> NOT YET NEEDED
• check what else we need

[markkovari]

watchdog be like