* Add Client4 route building functions
* Make DoAPIRequestWithHeaders add the API URL
This makes it consistent with the other DoAPIXYZ functions, which all
prepend the provided URL with the client's API URL.
* Use the new route building logic in Client4
* Address review comments
- clean renamed to cleanSegment
- JoinRoutes and JoinSegments joined in Join
- newClientRoute uses Join
* Fix new routes from merge
* Remove unused import
* Simplify error handling around clientRoute (#34870)
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Co-authored-by: Jesse Hallam <jesse@mattermost.com>
Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>
Co-authored-by: Mattermost Build <build@mattermost.com>
