feat: support raw JSON bodies in send step

[vitormattos]

Summary

• accept raw JSON request bodies in the generic Behat send step via PyStringNode
• preserve body-specific headers such as Content-Type: application/json
• keep the existing table-based behavior unchanged

Why

A consumer scenario in profile_fields needs to create workflow rules through the Nextcloud workflowengine OCS API using a nested JSON payload. Expressing that request directly in the feature file is more didactic than adding a domain-specific custom step, but the helper previously only supported tables and arrays.

Validation

• exercised from the consumer app scenario that posts to the workflowengine global workflows OCS endpoint with inline JSON
• consumer repo static analysis remained green after the change

Notes

This PR changes the shared helper only. The consumer app can update to the next released package version afterward.