fix(auth) improve error when passing a PEM key as ServiceAccountKey

[cgoetz-inovex]

Description

relates to STACKITSDK-280

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)

[Fyusel]

One thing: If we want to add this directly we need to increase the version of the core package and write changelogs.