Allow choosing the stack through the Build Trigger API

Description of the feature request

Adding the possibility to choose the Stack inside the Payload of the Build Trigger API.


 "build_params": {
    "branch": "master"
    "stack": "Xcode 9.3"

Use case / for what or how I would use it

Because of internal reasons we use Jenkins to trigger Bitrise builds via the Build Trigger API.
To prevent the need to go into bitrise and change the stack on every Xcode update, we would like our developers to choose the stack though a configuration inside the repository.

Definitely a great idea, thanks for the #feature-request! Don’t forget to vote on it to bump its priority! :wink: