Add convenience helper for associated-request derived components

[yaronf]

Summary

Response signatures often need to cover request-derived components using the ;req parameter (RFC 9421 §2.2.2), e.g. @method;req, @path;req, @query;req.

Today this requires AddHeaderExt with four booleans:

fields.AddHeaderExt("@method", false, false, true, false).
    AddHeaderExt("@path", false, false, true, false).
    AddHeaderExt("@query", false, false, true, false)

The signature is (optional, binarySequence, associatedRequest, trailer). For the WIMSE HTTP-Sig response case, associatedRequest=true and optional=false — these components are required, not optional.

Proposal

Add a convenience method:

// AddRequestComponent adds a required derived component from the associated request (sets ;req).
func (fs *Fields) AddRequestComponent(name string) *Fields

Usage:

fields.AddHeaders("@status", "workload-identity-token").
    AddHeaderOptional("Content-Type").
    AddHeaderOptional("Content-Digest").
    AddRequestComponent("@method").
    AddRequestComponent("@path").
    AddRequestComponent("@query")

Equivalent to:

AddHeaderExt(name, false, false, true, false)

No optional variant is proposed: WIMSE (and typical response-binding profiles) require these components whenever the response is signed. Optional coverage applies to headers like Content-Type / Content-Digest (AddHeaderOptional), not to @method;req / @path;req / @query;req.

Motivation

• Reduces caller mistakes (easy to get the four booleans wrong).
• Makes the required-vs-optional distinction explicit at the call site.
• WIMSE HTTP-Sig response signing uses this pattern for every signed response.

Related consumer: https://github.com/yaronf/wimse-s2s-doc