Add support local variables to message json body

[SoulAndris]

Hello. I had a need to run events, with the transfer of local variables, so that they were not visible on the main process.

There is such an opportunity in the documentation, I checked it with a postman, everything also works. I wanted to expand api through PR, but no returns 403 ((

link to documentation: https://docs.camunda.org/manual/7.11/reference/rest/message/post-message/