feat: add option to disable chunked headers

[irenarindos]

References to other Issues or PRs

n/a

Have you read the Contributing Guidelines?

Yes

Brief description of what is fixed or changed

Adds the option, WithDisableChunkedEncoding() to the ServeMux. If set, this will not set the "Transfer-Encoding": "chunked" header in ForwardResponseStream.

Other comments

[johanbrandhorst]

This looks great, thank you!

[johanbrandhorst]

I'll ignore the fuzz failure, it's unrelated.

[johanbrandhorst]

Thanks for your contribution ❤️! I just put out v2.27.8, like, yesterday, if you can test this for your needs and let me know how it goes I can put out another release in a week or two depending on what else comes in.

[irenarindos]

@johanbrandhorst Thanks so much, this works perfectly for us!