Add SearchIndex/AsyncSearchIndex serialization helpers

[tylerhutcherson]

Summary

Add serialization helpers for SearchIndex and AsyncSearchIndex to improve automation and reproducibility.

Level: Intermediate

Current State

• RedisVL has schema-centric factory methods, but limited top-level index serialization helpers.
• Agent and automation flows often need a portable index config artifact.

Proposed Change

• Add to_dict() and optional to_yaml() helpers for index configuration.
• Include schema and safe connection metadata with redaction options for sensitive fields.
• Ensure sync/async parity where applicable.

Definition of Done

• Round-trip config serialization is supported and tested.
• Sensitive connection details can be omitted/redacted.
• Existing index creation and usage remain non-breaking.

Out of Scope

• Full environment secret management framework.