Refactor ParsePublicURL

[gerardsn]

All URLs obtained from external sources (metadata endpoints) need to be validated for OAuth2/OpenID4VC meaning that all methods using these URLs will require the following codeblok:

if strictmode {
	issuerURL, err = core.ParsePublicURL(issuer, false, "https")
} else {
	issuerURL, err = core.ParsePublicURL(issuer, true, "https", "http")
}

This PR makes that a bit easier

[reinkrul]

Why "Public"?

[gerardsn]

Because this is effectively a validation of the auth.publicUrl config value configured in the other nuts node. Maybe a bit redundant, but I like that it is specific.