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

Original commit: 72a45fd9b7c7f21914a0a2d363e428b423db011b / D46273

Test Plan: Jenkins

Reviewers: sanketh, jhe, #db-approvers

Reviewed By: sanketh, #db-approvers

Subscribers: rthallam, yql, ybase

Differential Revision: https://phorge.dev.yugabyte.com/D46309