Fix the return type of Run.add to be array of ints

[lampajr]

Fixes Issue

The method Run.addRun was properly configured to return an array of ints but the implementation was returning it as String.

This is causing some issues on the auto generate Python client as it is expecting an array but getting a single element instead - The same problem does not happen in Golang as for some reason it is able to parse it anyway.

This method is not used anywhere in the source code, nor in the Jenkins plugin.

Changes proposed

• Simply return the List (with one single element though to avoid breaking the API)
• Do not add any header as not actually required (especially because not used)

Check List (Check all the applicable boxes)

• My code follows the code style of this project.
• My change requires changes to the documentation.
• I have updated the documentation accordingly.
• All new and existing tests passed.