fix: add --import-existing-resources to CDK deploy in integration-suite (#1174)

## What changed and why

Added `--import-existing-resources` flag to the `npx cdk deploy` command
in the `integration-suite` CI job
(`.gitlab/templates/pipeline.yaml.tpl`).

**Root cause**: When a pipeline fails mid-deploy, some AWS resources
(specifically the dotnet Lambda log group
`/aws/lambda/integ-{IDENTIFIER}-{TEST_SUITE}-dotnet-lambda`) are created
in AWS but the CloudFormation stack is left in `ROLLBACK_COMPLETE`
state. On a pipeline retry for the same commit (same `IDENTIFIER` =
`CI_COMMIT_SHORT_SHA`), CDK attempts to create the already-existing log
group and fails with:

```
ToolkitError: ChangeSet 'cdk-deploy-change-set' on stack 'integ-{IDENTIFIER}-{TEST_SUITE}' failed early validation:
  - Resource of type 'AWS::Logs::LogGroup' with identifier '/aws/lambda/integ-{IDENTIFIER}-{TEST_SUITE}-dotnet-lambda' already exists.
```

The `--import-existing-resources` flag tells CDK to import any
already-existing resources into the stack instead of failing validation,
which is the standard CDK solution for this exact scenario.


## Jira

[SVLS-8825](https://datadoghq.atlassian.net/browse/SVLS-8825)

[SVLS-8825]:
https://datadoghq.atlassian.net/browse/SVLS-8825?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: jchrostek-dd

.gitlab/templates/pipeline.yaml.tpl

@@ -508,7 +508,7 @@ integration-suite:
     - export CDK_DEFAULT_ACCOUNT=$(aws sts get-caller-identity --query Account --output text)
     - export CDK_DEFAULT_REGION=us-east-1
     - npm run build
-    - npx cdk deploy "integ-${IDENTIFIER}-${TEST_SUITE}" --require-approval never
+    - npx cdk deploy "integ-${IDENTIFIER}-${TEST_SUITE}" --require-approval never --import-existing-resources
     - echo "Running ${TEST_SUITE} integration tests with identifier ${IDENTIFIER}..."
     - export TEST_SUITE=${TEST_SUITE}
     - npx jest tests/${TEST_SUITE}.test.ts