fix: Preserve caching options after query split

[benclive]

What this PR does / why we need it:
Preserves the Caching options for LokiRequest objects through the splitting middleware.

• This makes the Cache-Control: no-cache header work as expected for queries that get split.
• Prior to this change, the header had no effect.

---

Note

Low Risk
Small, localized change that only copies an existing field into split sub-requests; behavior impact is limited to caching controls on split queries.

Overview
Preserves per-request CachingOptions when query splitting creates sub-requests, so cache-control settings (e.g. no-cache) are not lost after splitting.

This updates the splitter factories for LokiRequest (both default and metric query splitters) and logproto.VolumeRequest to copy CachingOptions, and adds a unit test ensuring split LokiRequests retain the original caching configuration.

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[salvacorts]

LGTM!