fix(iaas): read write only field correctly from the config model

[SerseusWasTaken]

Description

Fixes the write-only field KeyPayloadBase64WriteOnly by reading it from the config model instead of plan model.
Also adds a regression test based on mocking that verifies that the write only field is sent to the API.

Relates to STACKITTPR-782

Checklist

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