Implement ArrayAccess interface for Vector and HashMap

Summary:
Zed exposes the configuration as a hashmap to the site health checks:
private HashMap $config.

A check need the name of the database. ArrayAccess can allow this syntax:
$this->config['database']['database']

Ref T1678.

Test Plan: Unit tests provided.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1678

Differential Revision: https://devcentral.nasqueron.org/D2509