Add NewCertificateFromX509 function

[hslatman]

The new function can create a x509util.Certificate from an x509.Certificate (used as a template). It also supports options, to provide a template, so that data can be set in the certificate.

Currently a template is always required. An error will be returned when no template is provided.

@maraino: I'll add tests later if this approach seems sensible to you. I don't think we can use generics to handle both x509.Certificate and x509.CertificateRequest options using only a single implementation, so I copied them over.

[maraino]

I ❤️ the option of using generics.

[hslatman]

@maraino I made the options generic in 5199601. Haven't updated all usages yet to keep the diff readable. Does this look reasonable to you? We'll need to update usages of Options in dependent code. Should be fairly quick to do.

If the approach looks OK, I'll add some tests.

[hslatman]

@maraino I've gone ahead and updated the usages within crypto.

Do you think it's useful to provide helper functions like these:

func WithCSRTemplate(text string, data TemplateData) Option[*x509.CertificateRequest] {
	return WithTemplate[*x509.CertificateRequest](text, data)
}
// or:
func WithCertificateRequestTemplate(text string, data TemplateData) Option[*x509.CertificateRequest] {
	return WithTemplate[*x509.CertificateRequest](text, data)
}

Usage would then look a bit cleaner:

fn := WithCSRTemplate(tt.args.text, tt.args.data)

Instead of

fn := WithTemplate[*x509.CertificateRequest](tt.args.text, tt.args.data)

Defining aliases like the below is possible, but then requires defining the apply too:

type CertificateOptions Options[*x509.Certificate]

func (co *CertificateOptions) apply(t *x509.Certificate, opts []Option[*x509.Certificate]) (*Options[*x509.Certificate], error) {
	o := &Options[*x509.Certificate]{}
	return o.apply(t, opts)
}

Then another thing we could add is a type alias for a single CertificateOption (and CertificateRequestOption):

type CertificateOption Option[*x509.Certificate]

func (co *CertificateOptions) apply(t *x509.Certificate, opts []CertificateOption) (*Options[*x509.Certificate], error) {
	o := &Options[*x509.Certificate]{}
	var copts []Option[*x509.Certificate]
	for _, opt := range opts {
		copts = append(copts, Option[*x509.Certificate](opt))
	}
	return o.apply(t, copts)
}

// resulting in this function signature:
func NewCertificateFromX509(template *x509.Certificate, opts ...CertificateOption) (*Certificate, error)

Let me know if you think the above things would be useful, and which you would like to have available.

[hslatman]

Closing in favor of #248.