Bedrock: Prompt Management. Add ability to add variables for `system` prompt

### Describe the feature

Currently, there is a limitation in Prompt Management:
- it's impossible to use variables in the `system` part of the prompt

### Use Case

When defining a system prompt, most of the prompts I worked on involve a necessity to have variables in the `system` (role defining) part of the prompt.

### Proposed Solution

Add ability `to reuse` and `add unique` (used only in the `system` part) variables.
Than, expose it through the construct.

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

No file, test, or entry point is identified in the issue. Locate the Bedrock Prompt Management construct and its existing prompt-variable handling, then inspect related tests if present; done means variables can be reused or added uniquely in the system prompt and the capability is exposed through the construct.

Written by the indexing model from the issue text.