feat: Support setting federated token as env

[JorTurFer]

Description

This PR supports setting the federated token directly as env var. This is not really useful for long term workloads but it's quite interesting for CI processes (I need this to support Gitlab in terraform) and I think that adding the default behaviour to the SDK is better than adding the logic in multiple places.

Thanks to this change, if there is an env var STACKIT_FEDERATED_TOKEN in the system, it will be used as "raw"
token, if not, it'll look for STACKIT_FEDERATED_TOKEN_FILE as before. Any override done by the SDK caller is respected, this is just a change in default behaviour when missing extra configs

Checklist

• Issue was linked above
• No generated code was adjusted manually (check comments in file header)
• Changelogs
  • Changelog in the root directory was adjusted (see here)
  • Changelog(s) of the service(s) were adjusted (see e.g. here)
• VERSION file(s) of the service(s) were adjusted
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• 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)