Summary:
Add tserver GFlag counter part of yb_enable_cbo, a GUC parameter introduced by #26868.
Jira: DB-17048

Original commit: 006c7f38b160a297c3f547f26e4e87d2d62affd4 / D44518

Test Plan: Manual test with both valid and invalid values, jenkins

Reviewers: hsunder, mihnea

Reviewed By: hsunder

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D44686