Add conditional download caching to LoadRuleGroups

[PaurushGarg]

What this PR does:
Adds conditional download caching to LoadRuleGroups() in the bucket rule store. Instead of downloading every rule group file from object storage on each sync cycle, it checks if the file has changed (via HEAD/Attributes) before doing a full GET.

Why:
The ruler syncs rule groups from object storage every poll interval (default 60s). For tenants with hundreds of rule groups, downloading all files every cycle is wasteful — most files don't change between syncs. The full GET + protobuf unmarshal for each file adds up, causing sync duration to approach or exceed the poll interval and trigger missed syncs.

Flow:

• First sync (cold cache): normal GET for all files — no behavior change
• Subsequent syncs: HEAD to check LastModified → if file unchanged, return cached content (no GET, no unmarshal)
• Cache is pruned after each sync to remove entries for rule groups no longer owned by this pod
• If HEAD fails, falls through to normal GET

Observability:
New metric cortex_ruler_rule_group_load_cache_operations_total{result="hit|miss"} tracks cache effectiveness.

Testing:

• TestLoadRuleGroupsCache — verifies cache hit on unchanged files
• TestLoadRuleGroupsCacheMissOnModification — verifies modified files are re-downloaded
• TestLoadRuleGroupsCachePrune — verifies stale entries are cleaned up after ring rebalance

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]
• docs/configuration/v1-guarantees.md updated if this PR introduces experimental flags