The onefootball backend engineer task

To run the solution file run go run teams.go

To run tests run go test

1. Remove the first for loop in main() and replace it with something cleaner.
2. Proper error handling, possibly logging errors instead of just calling panic()
3. Synchronize writes to roster for better handling of concurrent writes.
4. Refactor functions to make them more testable and decrease dependency.
5. Write more tests.