add expiresAfter to use a relative delay for expires

[krichprollsch]

Hello,

First, thank you very much for the lib!

While using the wrap handler, I found useful to have way to configure a relative expiration delay instead of configuring an absolute one.

[yaronf]

Looks good. Two comments (thanks Cursor...)

1. Doc comment – The comment says the parameter is “as Unix time”, but it’s a delay in seconds. Suggested wording:

// SetExpiresAfter sets the "expires" parameter to createdTime + delay (seconds).
// Use this for a relative validity window instead of an absolute timestamp.
// Default: 0 (do not add the parameter).

2. Tests – No tests were added. Suggested additions:

• SetExpiresAfter(60) with setFakeCreated(1000) → expires = 1060
• Error when both SetExpires and SetExpiresAfter are set
• Optional: handler test using SetExpiresAfter instead of SetExpires(2000) for the early-expires case

[krichprollsch]

Thanks for the review and for your patience. I just updated my PR.

[yaronf]

Looks good, thanks for your contribution. I will follow up with a minor fix.