Added typing / comments to the SenderInterface

[danielmorell]

Description of the change

This PR adds better typing and comments to the SenderInterface to make implementation less error prone. I also updated our three sender classes to properly implement the interface.

I removed the SenderInterface::toString() method from the interface as well as all the classes that implement it. As best I can tell have not used the toString() method since we removed the internal debug logger in v1.6.3.

Type of change

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

Related issues

None

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 assigned
• Issue from task tracker has a link to this pull request
• Changes have been reviewed by at least one other engineer

[waltjones]

👍