Preserve raw response bytes in ResponseChain

[volksec]

Summary

• add an opt-in NewResponseChainWithRaw constructor
• preserve response bytes before content decoding and charset normalization
• expose safe raw-body and raw-full-response accessors
• keep the existing NewResponseChain allocation and behavior unchanged

Motivation

This is the dependency-level support needed by projectdiscovery/nuclei#7387. Nuclei currently receives only the normalized body from ResponseChain, so it cannot expose an encoded response payload alongside the existing decoded body.

The opt-in constructor keeps the additional pooled buffer out of callers that do not need raw bytes. Both raw and decoded reads remain bounded by the configured maximum body size.

Validation

• go test ./http
• go vet ./http
• regression test confirms a gzip response remains decoded through BodyString() while RawBodyBytes() preserves the original gzip stream