Fix fabric index management in the fabric table. (#16587)

* Fix fabric index management in the fabric table.

Before this change, the fabric index for a fabric table entry had to match its offset in the table.  Therefore we only used indices up to CHIP_CONFIG_MAX_FABRICS and then looped back around to index 1.

Furthermore, we did not use to persist our mNextAvailableFabricIndex,
so after a restart would start assigning fabric indices that
corresponded to deleted fabrics, instead of not-yet-used ones, even if
we had not yet exceeded CHIP_CONFIG_MAX_FABRICS total commissioning
events.

This change decouples the fabric index from the table offset.  We now
store the list of fabric indices used and the fabric index we should
use for the next time a fabric is added.

* Fix TestCASESession compilation.

* Address review comments.

Author: bzbarsky-apple