scion-pki: allow creating voting certificates without ISD-AS

[oncilla]

Only for cp-root, cp-ca, and cp-as certificates the ISD-AS is mandatory.

[Copilot]

Pull Request Overview

This PR modifies the certificate creation logic to make ISD-AS optional for voting certificates while keeping it mandatory for cp-root, cp-ca, and cp-as certificates.

Key Changes

• Added requireIA parameter to certificate subject creation functions to conditionally enforce ISD-AS presence
• Updated certificate creation to only require ISD-AS for Sensitive and Regular certificate types
• Enhanced test coverage to verify both ISD-AS required and optional scenarios

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
scion-pki/certs/create.go	Modified createSubject and loadSubject to accept requireIA parameter; updated certificate creation to conditionally require ISD-AS based on certificate type
scion-pki/certs/renew.go	Updated subjectFromVars to accept requireIA parameter and conditionally validate ISD-AS; passed true for renewal operations
scion-pki/certs/renew_test.go	Added RequireIA field to test cases and new test case for validating certificates without ISD-AS

Comments suppressed due to low confidence (1)

scion-pki/certs/renew.go:940

• When requireIA is false and vars.IA.IsZero() is true, the code still adds the ISD-AS to ExtraNames with an empty string value from vars.IA.String(). This will result in an invalid certificate attribute. The ExtraNames entry should only be added when vars.IA is not zero.

func subjectFromVars(vars SubjectVars, requireIA bool) (pkix.Name, error) {
	if requireIA && vars.IA.IsZero() {
		return pkix.Name{}, serrors.New("isd_as required in template")
	}
	s := pkix.Name{
		CommonName:   vars.CommonName,
		SerialNumber: vars.SerialNumber,
		ExtraNames: []pkix.AttributeTypeAndValue{
			{
				Type:  cppki.OIDNameIA,
				Value: vars.IA.String(),
			},
		},

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