Add optional retry policy

[jemc]

For cases where the underlying LLM fails to generate valid structured data, or the data generated doesn't match certain post-response validation requirements, it may be desirable to retry the generation, prompting the LLM with feedback about what it did wrong, including the schema validation errors.

This may not always be desirable (it can introduce extra cost or latency), so it should be optional.

The retry policy should ideally be implemented at the Kurt level (not the adapter level) so that all adapters can benefit from it without needing to introduce it as a feature for each adapter.