Control the visibility of invisible index in session level

[hawkingrei]

Enhancement

We already have "invisible index" implemented. However, variable use_invisible_indexes was not included. This could be caused by incomplete support of optimizer_switches.

As we are about to support index advisor in H1. This ability becomes important to verify the proposed indexes. Before implementing optimizer_switches, we could introduce a new variable in TiDB for this purpose.

• 1. Introduce variable tidb_opt_use_invisible_indexes whose scope is session only. *: support tidb_opt_use_invisible_indexes #50654