Update organizations endpoint in bitrise api to support velocity plan

Description of the feature request

Hi there,

it would be cool if the existing organizations endpoint would provide information about the available concurrency count or even the available and used credits.
It actually contains the field concurrency_count but since we switched to the velocity plan it is always null :frowning:

Use case / for what or how I would use it

We use the Bitrise api to display a dashboard with the status of the latest builds and the current build queue traffic within a native macOS app. This helps us to get a quick overview about the build queue.
We would love to have a quick overview of the current traffic on the build queue as well.