logging: add set_cookie log filter for Set-Cookie response headers

[steffenbusch]

Closes #7887

This adds a new set_cookie log field filter for logged Set-Cookie response header values.

It supports the same actions as the existing cookie filter:

• hash
• replace
• delete

The main intended use is for access log fields such as resp_headers>Set-Cookie when log_credentials is enabled.

Motivation

Caddy already has a built-in cookie log filter for logged Cookie request header values.

That logic does not map directly to Set-Cookie, because Set-Cookie is structured as one cookie plus attributes per header value. Reusing request-cookie-style handling can produce partially useful output, but does not preserve Set-Cookie attributes correctly.

Implementation notes

This implementation uses a hybrid approach:

• it uses http.ParseSetCookie() to identify the cookie name and value
• it does not re-serialize the full header with Cookie.String()
• it preserves the original attribute suffix verbatim
• it only transforms the leading cookie value
• invalid / unparseable Set-Cookie lines are passed through unchanged

This avoids normalization and preserves unrecognized attributes.

Example

log {
	format filter {
		fields {
			resp_headers>Set-Cookie set_cookie {
				hash session
				delete csrf
			}
		}
	}
}

Tests

Added tests for:

• hash
• replace
• delete
• first-match-wins behavior
• invalid header passthrough
• case-sensitive cookie-name matching
• Set-Cookie values without attributes
• empty cookie value hashing
• preserving quoted values / suffix formatting / unknown attributes

Assistance Disclosure

ChatGPT/Codex was used to help draft the issue/PR text, refine the implementation approach, and assist with code and test changes. The overall feature idea and final decisions on the technical approach and submitted content were mine.