feature: support rich HTTP scenarios (method, headers, body)

[marr97]

• Extend config schema to describe a request (method, url, headers, body)
• Update agent HTTP sender to send non-GET requests and include headers/body
• Validate scenario config (method supported, body allowed, url required)
• Add examples (config.json / config.yaml) for POST/PUT with JSON body
• Add tests for config parsing and request building