Add command line options to http3_client for client certs

Make it possible to use client TLS certificates with the HTTP/3 client
demo.

Fixes: #498

examples/http3_client.py

@@ -437,6 +437,11 @@ async def main(
     parser.add_argument(
         "--ca-certs", type=str, help="load CA certificates from the specified file"
     )
+    parser.add_argument(
+        "--certificate",
+        type=str,
+        help="load the TLS certificate from the specified file",
+    )
     parser.add_argument(
         "--cipher-suites",
         type=str,
@@ -482,6 +487,11 @@ async def main(
         type=str,
         help="write downloaded files to this directory",
     )
+    parser.add_argument(
+        "--private-key",
+        type=str,
+        help="load the TLS private key from the specified file",
+    )
     parser.add_argument(
         "-q",
         "--quic-log",
@@ -559,6 +569,10 @@ async def main(
         except FileNotFoundError:
             pass
 
+    # load SSL certificate and key
+    if args.certificate is not None:
+        configuration.load_cert_chain(args.certificate, args.private_key)
+
     if uvloop is not None:
         uvloop.install()
     asyncio.run(