chore(ci): adding datadog-lambda-python CI to Gitlab (#15475)

## Description
Adding the unit and integration tests from the datadog-lambda-python
repository to this repository's CI. Helps catch any changes to
dd-trace-py that would cause issues in datadog-lambda-python before
merging/release.

<!-- Provide an overview of the change and motivation for the change -->

## Testing
Gitlab. 
Verified that if trigger-serverless-lambda-tests downstream pipeline
fails, the whole ddtrace pipeline will fail and the
dd-gitlab/default-pipeline job will show as failed in the PR using [this
Gitlab
pipeline](https://gitlab.ddbuild.io/DataDog/apm-reliability/dd-trace-py/-/pipelines/86791461).
<!-- Describe your testing strategy or note what tests are included -->

## Risks

<!-- Note any risks associated with this change, or "None" if no risks
-->

## Additional Notes
[Related PR in
datadog-lambda-python](DataDog/datadog-lambda-python#700).
<!-- Any other information that would be helpful for reviewers -->

Author: rithikanarayan

.gitlab-ci.yml

@@ -75,6 +75,26 @@ run-tests-trigger:
        job: tests-gen
    strategy: depend
 
+serverless lambda tests:
+  stage: tests
+  trigger:
+    project: DataDog/datadog-lambda-python
+    strategy: depend
+    branch: main
+  needs:
+    - job: publish-wheels-to-s3
+  variables:
+    UPSTREAM_PIPELINE_ID: $CI_PIPELINE_ID
+    UPSTREAM_PROJECT_URL: $CI_PROJECT_URL
+    UPSTREAM_COMMIT_BRANCH: $CI_COMMIT_BRANCH
+    UPSTREAM_COMMIT_AUTHOR: $CI_COMMIT_AUTHOR
+    UPSTREAM_COMMIT_TITLE: $CI_COMMIT_TITLE
+    UPSTREAM_COMMIT_TAG: $CI_COMMIT_TAG
+    UPSTREAM_PROJECT_NAME: $CI_PROJECT_NAME
+    UPSTREAM_GITLAB_USER_LOGIN: $GITLAB_USER_LOGIN
+    UPSTREAM_GITLAB_USER_EMAIL: $GITLAB_USER_EMAIL
+    SKIP_E2E_TESTS: "true"
+
 # Validate the ast-grep rule's test suite in .sg/tests
 "ast-grep rules":
   extends: .testrunner