Summary:
Default hard coded timeout for DDL RPCs like CreateTable of 60s is too low for xCluster like cases. Bumping the value to 180s and adding gFlag --ysql_ddl_rpc_timeout_sec so that it can be configured at runtime.
Jira: DB-17816

Test Plan: Jenkins

Reviewers: sanketh, jhe

Reviewed By: sanketh

Subscribers: ybase, yql

Differential Revision: https://phorge.dev.yugabyte.com/D46273