Add constructor for static key strategy

Co-authored-by: Josh Winters <jwinter@pivotal.io>
Signed-off-by: Rui Yang <ruiya@vmware.com>

server/server.go

@@ -2,6 +2,7 @@ package server
 
 import (
 	"context"
+	"crypto/rsa"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -172,6 +173,13 @@ func NewServer(ctx context.Context, c Config) (*Server, error) {
 	))
 }
 
+// NewServerWithKey constructs a server from the provided config and a static signing key.
+func NewServerWithKey(ctx context.Context, c Config, privateKey *rsa.PrivateKey) (*Server, error) {
+	return newServer(ctx, c, staticRotationStrategy(
+		privateKey,
+	))
+}
+
 func newServer(ctx context.Context, c Config, rotationStrategy rotationStrategy) (*Server, error) {
 	issuerURL, err := url.Parse(c.Issuer)
 	if err != nil {