add new Matrix Hookshot alerter

[jertel]

Description

Provides new alerter for integrating to Matrix server via Hookshot

Checklist

• [x] I have reviewed the contributing guidelines.
• [x] I have included unit tests for my changes or additions.
• [x] I have successfully run make test-docker with my changes.
• [x] I have manually tested all relevant modes of the change in this PR.
• [x] I have updated the documentation.
• [x] I have updated the changelog.

Questions or Comments

[nsano-rururu]

The following has been forgotten in the schema.yaml

 matrixhookshot_ca_certs: {type: [boolean, string]}