feat(benchmarks): add apm-sdk-benchmarks trigger for PHP competitor benchmarks (LANGPLAT-883)

Adds a manual trigger job in the benchmarks child pipeline that launches
the PHP competitor benchmark in DataDog/apm-reliability/apm-sdks-benchmarks.

Key differences from dd-trace-py's apm-sdk-benchmarks job:
- No `needs:` prerequisite: dd-trace-php has no per-pipeline S3 upload
  (unlike dd-trace-py which uploads wheels keyed by CI_PIPELINE_ID).
  The apm-sdks-benchmarks fetch script uses the latest GitHub release.
- Uses TARGET_REF: $PARENT_PIPELINE_ID (not CI_PIPELINE_ID): benchmarks.yml
  is a child pipeline, so CI_PIPELINE_ID is the child ID. PARENT_PIPELINE_ID
  is forwarded from .gitlab-ci.yml and identifies the main pipeline.

Author: Leiyks

.gitlab/benchmarks.yml

@@ -258,3 +258,21 @@ notify-slo-breaches:
   rules: *pre-release-performance-quality-gates-rules
   variables:
     CHANNEL: "guild-dd-php"
+
+apm-sdk-benchmarks:
+  stage: benchmarks
+  when: manual
+  allow_failure: true
+  # No `needs:` here — unlike dd-trace-py which uploads per-pipeline wheels to
+  # S3 (keyed by CI_PIPELINE_ID), dd-trace-php publishes to S3 keyed by VERSION
+  # string only. The apm-sdks-benchmarks fetch script downloads the latest
+  # GitHub release instead of a specific pipeline build.
+  trigger:
+    project: DataDog/apm-reliability/apm-sdks-benchmarks
+    branch: main
+  variables:
+    # PARENT_PIPELINE_ID (not CI_PIPELINE_ID): benchmarks.yml runs as a child
+    # pipeline, so CI_PIPELINE_ID here is the child pipeline ID. PARENT_PIPELINE_ID
+    # is explicitly forwarded from the parent pipeline in .gitlab-ci.yml and
+    # identifies the main dd-trace-php pipeline that triggered this run.
+    TARGET_REF: $PARENT_PIPELINE_ID