I hope postcat could support mock response header

### Describe the feature

I hope postcat could support mock response header, like I wanna attach "set-cookies" in header section. Currently postcat only supports mock response body

### Environment

ZSBD

No file or test is named. Start by locating the existing mock response body implementation and inspect how response headers are represented; confirm how a header such as set-cookie should be entered and returned. Done means mock responses can include custom response headers while existing body mocking continues to work.

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