Add tests for tenant identifier.

[bkford]

Adds 3 tests that ensure tenant identifier shows up correctly:

• Setting and accessing via dialplan
• Setting in pjsip.conf and presence in AMI events
• Presence in ARI events

Also added a new directory (extra_gates) with a symbolic link to the tenant_id tests to run gates beyond the normal ones for Asterisk reviews.

[github-actions[bot]]

REMINDER: If this PR applies to other branches, please add a comment with the appropriate "cherry-pick-to" headers as per the [Create a Pull Request](https://docs.asterisk.org/Development/Policies-and-Procedures/Code-Contribution) process.

If you don't want it cherry-picked, please add a "cherry-pick-to: none" comment so we don't keep asking.

If, after adding "cherry-pick-to" comments, you change your mind, please edit the comment to DELETE the header lines and add "cherry-pick-to: none".

[bkford]

cherry-pick-to: 22 cherry-pick-to: 21 cherry-pick-to: 20 cherry-pick-to: certified/20.7 cherry-pick-to: 18 cherry-pick-to: certified/18.9

[github-actions[bot]]

Successfully merged to branch master and cherry-picked to ["22","21","20","certified/20.7","18","certified/18.9"]