[Voyager] Add GRANT SQL for setting session_replication_role for PG 15+ and document the super user privilege for import data to source

[priyanshi-yb]

Jira Link: DB-15294

Description

In the fallback workflow, for the import data to source command, after cutover to target, we ask users to disable triggers and drop FK constraints on the source.
To avoid this manual effort of disable or drop the trigger and FKs and then re-enable or recreate them after cutover to source, import data to source command can set the session_replication_role if it has privileges that will disable the triggers and FK internally.
On PostgreSQL 15+, there is a GRANT statement that will grant the use of this session parameter to any user.

GRANT SET ON PARAMETER session_replication_role to <ybvoyager_user>;

But on PostgreSQL < 15, there is only superuser privilege which will allow using this session parameter.

Issue Type

kind/enhancement

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