out_chronicle: support the new Chronicle API

**Is your feature request related to a problem? Please describe.**
Google SecOps is deprecating the legacy Backstory API (including the Customer Management API) and the legacy Ingestion API in favor of the [Chronicle API](https://docs.cloud.google.com/chronicle/docs/reference/rest). For details on how to migrate, see [Migrate from legacy SIEM API to Chronicle API](https://docs.cloud.google.com/chronicle/docs/administration/migrate-from-legacy-api-to-chronicle-api).

Key dates:
- October 26, 2026: End of sale (EOS): New instances provisioned on or after this date cannot access the legacy APIs.
- July 20, 2027: End of life (EOL): The legacy APIs shut down. Existing instances must complete migration before this date.

Currently, `out_chronicle` supports Backstory API only.

**Describe the solution you'd like**
`out_chronicle` support both Backstory API and Chronicle API during transition phase, then decom the legacy Backstory API.

**Describe alternatives you've considered**
N/A

**Additional context**
- https://docs.cloud.google.com/chronicle/docs/deprecations

Start with the existing out_chronicle implementation and the linked Chronicle API reference and migration documentation. Done means the output supports both Backstory API and Chronicle API during the transition, with a later path to deprecating the legacy API.

Written by the indexing model from the issue text.