Enable sockets extension

Summary:
The sockets extension is used by php-amqplib, used by keruald/broker,
and so by Notifications Center to connect to RabbitMQ.

Jenkins PHP agents to run CI jobs for Notifications Center uses
this Docker image as base, so the sockets extension has to be added
in one of those two repositories.

Fedora PHP maintainers include sessions in the standard extensions package
php-common, so it's probably a good candidate here.

Reference: https://github.com/php-amqplib/php-amqplib/issues/608

Ref T1800

Test Plan: Run Notifications CI job

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1800

Differential Revision: https://devcentral.nasqueron.org/D2916