Summary:
This diff introduces a new flag `cdc_max_virtual_wal_per_tserver` to limit the number of VirtualWAL instances per tserver. When a new client tries to initialise a VirtualWAL, it will error out if we already have max VWAL instances on the tserver. This will help increase the system stability in the worst case scenario.
Jira: DB-15204

Test Plan: Jenkins: urgent

Reviewers: sumukh.phalgaonkar, skumar

Reviewed By: sumukh.phalgaonkar

Subscribers: sumukh.phalgaonkar, ybase, ycdcxcluster

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D41694