feat: support for key/certificate types RSA, Ed25519, ECDSA

This also automatically derives key type and certificate signature
algorithm when possible. Option to select signature algorithm and key
type should only be passed usually when generating new CA or key.

Key/cert types:

* default - Ed25519
* `x509.RSA(true)` - RSA
* `x509.ECDSA(true)` - ECDSA

Signed-off-by: Andrey Smirnov <smirnov.andrey@gmail.com>

x509/constants.go

@@ -8,3 +8,16 @@ import "time"
 
 // DefaultCertificateValidityDuration is a default certificate lifetime.
 const DefaultCertificateValidityDuration = 24 * time.Hour
+
+// PEM Block Header Types.
+const (
+	PEMTypeRSAPrivate     = "RSA PRIVATE KEY"
+	PEMTypeRSAPublic      = "PUBLIC KEY"
+	PEMTypeECPrivate      = "EC PRIVATE KEY"
+	PEMTypeECPublic       = "EC PUBLIC KEY"
+	PEMTypeEd25519Private = "ED25519 PRIVATE KEY"
+	PEMTypeEd25519Public  = "ED25519 PUBLIC KEY"
+
+	PEMTypeCertificate        = "CERTIFICATE"
+	PEMTypeCertificateRequest = "CERTIFICATE REQUEST"
+)