Use 'bytes' instead of 'string' for holding binary data

Proto 'string' fields are required to contain valid UTF-8 data, while 'bytes' fields are allowed to contain arbitrary data.  Since both proto types are expressed in C++ the same way, this should have no effect other than stopping this warning message:

E0325 09:52:56.156763    4987 wire_format_lite.cc:625] String field 'quic.QuicServerConfigProtobuf.PrivateKey.private_key' contains invalid UTF-8 data when serializing a protocol buffer. Use the 'bytes' type if you intend to send raw bytes.

gfe-relnote: No-op proto change, not flag-protected.
PiperOrigin-RevId: 240184602
Change-Id: Ib77b16245c21eef473160a39e9b777611d3899c5

quic/core/proto/crypto_server_config.proto

diff --git a/quic/core/proto/crypto_server_config.proto b/quic/core/proto/crypto_server_config.proto
index 06db4a7..bb447dc 100644
--- a/quic/core/proto/crypto_server_config.proto
+++ b/quic/core/proto/crypto_server_config.proto
@@ -19,7 +19,7 @@
   // private key is specific to the algorithm in question.
   message PrivateKey {
     required uint32 tag = 1;
-    required string private_key = 2;
+    required bytes private_key = 2;
   }
   repeated PrivateKey key = 2;