Add support for SSH certificates to OIDC.

Update the interface for all the provisioners.

authority/provisioner/method.go

@@ -0,0 +1,34 @@
+package provisioner
+
+import (
+	"context"
+)
+
+// Method indicates the action to action that we will perform, it's used as part
+// of the context in the call to authorize. It defaults to Sing.
+type Method int
+
+// The key to save the Method in the context.
+type methodKey struct{}
+
+const (
+	// SignMethod is the method used to sign X.509 certificates.
+	SignMethod Method = iota
+	// SignSSHMethod is the method used to sign SSH certificate.
+	SignSSHMethod
+	// RevokeMethod is the method used to revoke X.509 certificates.
+	RevokeMethod
+)
+
+// NewContextWithMethod creates a new context from ctx and attaches method to
+// it.
+func NewContextWithMethod(ctx context.Context, method Method) context.Context {
+	return context.WithValue(ctx, methodKey{}, method)
+}
+
+// MethodFromContext returns the Method saved in ctx. Returns Sign if the given
+// context has no Method associated with it.
+func MethodFromContext(ctx context.Context) Method {
+	m, _ := ctx.Value(methodKey{}).(Method)
+	return m
+}