Update test certs (#1522)

Motivation:

Test certs expired; tests now fail.

Modifications:

- Add a script to generate a Swift source file which uses the output of
  the existing `makecert` script.
- Fix a warning.

Results:

Tests pass.

Author: glbrntt

.swiftformat

@@ -3,6 +3,7 @@
 
 # Ignore generated files.
 --exclude **/LinuxMain.swift,**/XCTestManifests.swift,**/*.grpc.swift,**/*.pb.swift
+--exclude Sources/GRPCSampleData/GRPCSwiftCertificate.swift
 
 --indent 2
 --maxwidth 100

Sources/GRPC/Serialization.swift

@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+import struct Foundation.Data
 import NIOCore
 import NIOFoundationCompat
 import SwiftProtobuf