Add support for custom status code in TimeoutLayer

[FalkWoldmann]

Motivation

• See Change Timeout middleware to return HTTP 503 Service Unavailable status code #579 (comment)
• Partially fixes Should TimeoutLayer middleware allow returning other status codes? #300

Solution

• Implement `TimeoutLayer::with_status_code'
• Mark TimeoutLayer::new as deprecated

[jplatte]

Thanks for the PR!

[FalkWoldmann]

Hi @jplatte, sorry to ping you, but would you be able to check this PR again? I would like to use this feature soon :)

[jplatte]

Always fine to ping me after a week or two of inactivity :)
I'll try to get to this soon.

[jplatte]

Thanks for your patience! Do you need this in a release?

[FalkWoldmann]

Thanks for the review :) a release would be cool, since we can get rid of some custom code, but it's not urgent

[frederikhors]

I don't understand why the deprecation of the new if the default StatusCode::REQUEST_TIMEOUT is ok for most of people.

[teohhanhui]

@frederikhors #579 (comment)

I don't think HTTP 408 Request Timeout is an okay default at all... The closest thing to a sensible default would be HTTP 503 Service Unavailable.