Export methods to allow generating custom certificates

[peterargue]

Closes: libp2p/go-libp2p#1538

This PR refactors keyToCertificate into 3 methods

• DefaultCertTemplate (exported), which returns the default certificate template used to generate the self-signed certificate,
• generateSignedExtension, which generates the signed extension
• keyToCertificate, which accepts an x509.Certificate template, adds the signed extension and returns the tls.Certificate object.

This PR also adds a new optional parameter to NewIdentity, which can provide a custom certificate template.

The changes are intended to be backwards compatible and add the ability for a caller to specify the x509 certificate fields they wish to include in the generated certificate.

[marten-seemann]

As discussed before, the missing issuer is a separate issue that will be solved by #100. Do I understand correctly that once that PR is merged, you'll only need to expose a single method?

[peterargue]

yea, we only really need the GenerateSignedExtension method exposed. if needed, we could implement our own custom certificates as long as we could generate the extension.

[peterargue]

If you'd prefer not to export the methods, I could refactor this to accept options that optionally set a template. that would avoid exporting any methods and keep the api backwards compatible.
e.g.

func NewIdentity(privKey ic.PrivKey, opts ...IdentityOption) (*Identity, error) {
	config := IdentityConfig{}
	for _, opt := range opts {
		opt(&config)
	}

	var err error
	if config.CertTemplate == nil {
		config.CertTemplate, err = defaultCertTemplate()
		if err != nil {
			return nil, err
		}
	}

	cert, err := keyToCertificate(privKey, config.CertTemplate)
	if err != nil {
		return nil, err
	}

[peterargue]

@marten-seemann does this look ok?

[marten-seemann]

@peterargue We just moved go-libp2p-tls into go-libp2p (libp2p/go-libp2p#1466). Could you re-target this PR to go-libp2p please? Sorry for the inconvenience!

[peterargue]

for sure. no problem

[MarcoPolo]

Closing this as it's handled already in libp2p/go-libp2p#1481