[#11693] Fix partition_key to use bytes

Summary:
partition_key was added to PgsqlWriteRequestPB in recent c5f5125/D13244 commit.
The field has type string, but it could contain arbitrary data, so bytes type should be used for it.
This diff fixes the issue by switching to bytes type.

Test Plan: Jenkins

Reviewers: dmitry

Reviewed By: dmitry

Subscribers: bogdan, ybase

Differential Revision: https://phabricator.dev.yugabyte.com/D15877

src/yb/common/pgsql_protocol.proto

@@ -257,7 +257,7 @@ message PgsqlWriteRequestPB {
   repeated PgsqlColRefPB col_refs = 21;
 
   // Used only in pg client.
-  optional string partition_key = 22;
+  optional bytes partition_key = 22;
 }
 
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