feat: add support for reading request body from file

[11eshaannegi]

Adds support for reading the request body from a file using the -D (--body-file) flag. This simplifies handling large or complex payloads that are difficult to pass via CLI.

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Changes

• Added -D, --body-file <file_path> flag to read request body from a file
• Integrated file input into existing request flow
• Maintained compatibility with existing -b flag
• Ensured mutual exclusion between -b and -D
• Added validation for invalid or missing files

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Usage

goperf run -m POST -D payload.json https://httpbin.org/post

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Why

Previously, request bodies could only be passed using -b, which becomes inconvenient for large JSON payloads. This improves usability and supports real-world scenarios.

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Testing

• Tested with sample JSON file (`payload.json`)
• Verified correct request body is sent

[rajat-tomar]

Mutual exclusion check and error wrapping are good but three things to address:

1. Config file merge is incomplete — see inline comment on configfile.go
2. Needs tests. config_test.go already has a table-driven Validate suite with the validConfig() helper. Add cases for: both -b and -D set (should error), nonexistent file (should error), valid file (Body should be populated after Validate)
3. Fix the usage example in this PR description — goperf.exe run -- POST -D payload.json won't work. The -- means end-of-flags, so POST becomes the URL argument. Should be: goperf run -m POST -D payload.json https://httpbin.org/post

[github-actions]

🎉 This PR is included in version 1.11.0 🎉

The release is available on GitHub release

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