v3.2: Provide guidance for the Set-Cookie response header

[handrews]

Partially addresses issue #1237

The Set-Cookie response header breaks the normal rules for headers with multiple values and requires special handling.

There are two options here:

1. Rework how we handle headers to accommodate Set-Cookie in a consistent manner
2. Treat Set-Cookie as a special case and define how it relates to normal behavior

Since RFC9110 §5.3 advises clients to treat Set-Cookie as a special case, I went with special-casing it. This assumes the approach to header serialization that is described in PR #4648 and discussed in one of its comment threads (specifically, that it does not include the header name which is required for it to be consistent with how style: "simple" and explode are treated elsewhere and are defined in RFC6570).

• schema changes are included in this pull request
• schema changes are needed for this pull request but not done yet
• no schema changes are needed for this pull request