Expose CourierClientInterface implemented by CourierClient for easier consumer testing

[tk26]

Description of the change

Expose CourierClientInterface, which is implemented by CourierClient.

Type of change

• [ ] Bug fix (non-breaking change that fixes an issue)
• [x] New feature (non-breaking change that adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Related issues

Fix [#1]()

Checklists

Development

• [ ] Lint rules pass locally
• [ ] The code changed/added as part of this pull request has been covered with tests
• [ ] All tests related to the changed code pass in development

Code review

• [ ] This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• [ ] "Ready for review" label attached to the PR and reviewers mentioned in a comment
• [ ] Changes have been reviewed by at least one other engineer
• [ ] Issue from task tracker has a link to this pull request

[linear[bot]]

C-3668 Benevity: PHP SDK suggestion for better interfacing

From Benevity's `@mevans` - "Here's a suggestion for the PHP SDK. `CourierClient` is a final class, which makes it hard to mock using our mocking tools (Mockery). To get around this, I have defined `CourierClientInterface` and a `CourierClientWrapper` that implements the interface. `CourierClientInterface` defines the same `sendNotification` method as `CourierClient`. The wrapper's constructor accepts an instance of `CourierClient` and delegates its implemented `sendNotification` method calls to it. Then I can mock `CourierClientInterface` and create the needed test double without PHP raising errors at me. If instead the SDK provided an interface and the final class implemented it, then SDK consumers could use and mock that interface without the need for the wrapper.Food for thought. I'm not blocked."