feat(auth): add flag to choose used port in user auth flow

[marceljk]

Description

relates to STACKITCLI-276

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see e.g. here)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[github-actions]

Merging this branch changes the coverage (2 decrease, 1 increase)

Impacted Packages	Coverage Δ	🤖
github.com/stackitcloud/stackit-cli/internal/cmd/auth/login	55.56% (+55.56%)	🌟
github.com/stackitcloud/stackit-cli/internal/pkg/auth	56.36% (-0.40%)	👎
github.com/stackitcloud/stackit-cli/internal/pkg/flags	57.27% (-1.88%)	👎

---

Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/stackitcloud/stackit-cli/internal/cmd/auth/login/login.go	55.56% (+55.56%)	18 (+11)	10 (+10)	8 (+1)	🌟
github.com/stackitcloud/stackit-cli/internal/pkg/auth/auth.go	51.40% (ø)	107	55	52
github.com/stackitcloud/stackit-cli/internal/pkg/auth/user_login.go	0.00% (ø)	154 (+5)	0	154 (+5)
github.com/stackitcloud/stackit-cli/internal/pkg/auth/user_token_flow.go	86.41% (ø)	103	89	14
github.com/stackitcloud/stackit-cli/internal/pkg/flags/flag_to_value.go	24.00% (-1.81%)	100 (+7)	24	76 (+7)	👎

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/stackitcloud/stackit-cli/internal/cmd/auth/login/login_test.go
• github.com/stackitcloud/stackit-cli/internal/pkg/auth/auth_test.go
• github.com/stackitcloud/stackit-cli/internal/pkg/auth/user_token_flow_test.go