[ML] Add CI build timing analytics and Gradle build cache for Java ITs (#2907)

Adds a post-build step to the branch/snapshot pipeline that analyses
build+test timings for each platform, compares against the last 14
builds, and creates a Buildkite annotation flagging regressions
(>2 std deviations above mean) or improvements (>1 std dev below).

The step depends on all build+test jobs, uses allow_dependency_failure
and soft_fail so it never blocks the pipeline, and runs on a
lightweight python:3-slim Docker image.

Requires a Buildkite API token with read_builds scope stored in
Vault at secret/ci/elastic-ml-cpp/buildkite/api_read_token.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: edsavage

.buildkite/hooks/post-checkout

@@ -42,6 +42,7 @@ if [[ "$BUILDKITE_PIPELINE_SLUG" == ml-cpp* ]]; then
       echo "$SCCACHE_GCS_KEY_JSON" > "$SCCACHE_GCS_KEY_FILE"
       export GOOGLE_APPLICATION_CREDENTIALS="$SCCACHE_GCS_KEY_FILE"
       export SCCACHE_GCS_KEY_PATH="$SCCACHE_GCS_KEY_FILE"
+      export GRADLE_BUILD_CACHE_GCS_BUCKET="elastic-ml-cpp-sccache"
     fi
   fi