Support sfn->sfn context injection case 3

[lym953]

What and why?

Skip the context injection in Step Function execution step if the CONTEXT field exists and it's not an object.

This is case # 2 in this design doc: Fixing Step Function Instrumentation

Review checklist

• [x] Feature or bugfix MUST have appropriate tests (unit, integration)

[datadog-datadog-prod-us1[bot]]

Datadog Report

Branch report: yiming.luo/fix-step-func-15 Commit report: 1285c5a Test service: datadog-ci-tests

:white_check_mark: 0 Failed, 432 Passed, 0 Skipped, 1m 32.78s Total duration (2m 1.11s time saved)