[Analytics] Wire liquid cache into the analytics engine (plugin + integration)

[cocosz]

Description

Turns the liquid cache on: wires the vendored library into the analytics engine and exposes it as an OpenSearch plugin, gated behind the liquid_cache cargo feature and the datafusion.liquid_cache feature flag.

Because the earlier PRs already landed their files in final form, this PR is purely additive plus two small wiring flips:

• Enable the dependency: uncomments the optional opensearch-liquid-cache path dependency in the analytics backend Cargo.toml, and adds the top crate as a member of the liquid-cache workspace. The engine hook (liquid_cache.rs) is not touched here — it already shipped in its final form in [Analytics] Add optional liquid cache seam for indexed parquet scans (cargo feature) #22553.
• Top crate (Rust): adds ffi.rs, lib.rs, runtime.rs — the process-global runtime, engagement policy, and the lc_* C FFI.
• Plugin (Java): LiquidCachePlugin bootstraps the runtime, LiquidCacheBridge binds the lc_* symbols, LiquidCacheSettings exposes configuration, and LiquidCacheClearAction / LiquidCacheStatsAction provide admin/observability endpoints.
• Build + tests: plugin build.gradle builds the engine with -PliquidCache; LiquidCacheIT verifies an indexed match+predicate query engages the cache (total_entries > 0); FeatureFlags.java registers the flag.

This is PR 3 of a 3-part stack:

1. Seam ([Analytics] Add optional liquid cache seam for indexed parquet scans (cargo feature) #22553): feature-gated in-process hook (no-op by default).
2. Library ([Analytics] Vendor the liquid-cache library crates #22569): vendors the liquid-cache crates in isolation.
3. This PR — integration: turns the feature on + Java plugin + integration test.

Changes

Area	Files
Wiring flips	analytics Cargo.toml (uncomment dep), liquid-cache src/main/rust/Cargo.toml (add top-crate member)
Engine	analytics session_context.rs, DataFusionPlugin.java
Top crate	liquid-cache/src/main/rust/src/{ffi.rs, lib.rs, runtime.rs}
Java plugin	LiquidCachePlugin, LiquidCacheBridge, LiquidCacheSettings, action/LiquidCacheClearAction, action/LiquidCacheStatsAction
Build	plugin build.gradle, README.md, qa build.gradle
Feature flag	server/.../FeatureFlags.java
Tests	LiquidCacheSettingsTests, LiquidCacheIT

18 files changed, +1,606 / -18.

Check List

• Builds with -PliquidCache.
• LiquidCacheIT passes — indexed match+predicate query engages the cache (total_entries > 0).
• Default build (feature off) unaffected.
• API changes companion PR raised (n/a).
• Public documentation issue/PR created (n/a — sandbox).

Note: PRs target main, so until the parents merge this PR's GitHub diff includes the seam + library commits. Review/merge order: #22553 → #22569 → this PR.

[github-actions]

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