### What problem does this PR solve?
Bugfix: usage of Bedrock models require the system prompt (for models
that support it) to be provided in the API in a different way, at least
that was my experience with it just today. This PR fixes it.
https://docs.aws.amazon.com/bedrock/latest/userguide/conversation-inference.html
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)