Enable send event routing

[raniero-sequence]

Description of the change

This PR updates the SendInterface and its implementation in order to make calls to the /send/:event/routing endpoint. It adds request and response model classes in order to make requests via the SDK. I've added an example to the CourierClientApp class.

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

[tk26]

@raniero-sequence we will be supporting this feature via https://github.com/trycourier/courier-java/pull/18/files (pretty much the same as this PR with a couple of additional changes) Thanks again for the contribution :)