New API Endpoints Support: Add support for the Matrix Project Integration
## New API / Endpoint
Add support for the Matrix Project Integration API.
API documentation: https://docs.gitlab.com/api/project_integrations/#matrix-notifications
## Implementation Guide
The code in `client-go` has a strong pattern that is repeatable when adding support for new APIs.
- Read the instructions in [CONTRIBUTING.md](https://gitlab.com/gitlab-org/api/client-go/-/blob/main/CONTRIBUTING.md) to get your local development environment set up and clone the community fork of this project.
- Follow the instructions in [AddingAPISupport.md](https://gitlab.com/gitlab-org/api/client-go/-/blob/main/docs/guides/AddingAPISupport.md).
- This file includes instructions for adding all endpoints of an API.
- It also describes how to write functions for each type of endpoint, so can be used for adding new endpoint support as well.
- This will be added to the existing `services.go` and `services_test.go` files.
- Run `make reviewable` before raising an MR.
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