Expose the default branch setting to the CLI

[fabianegli]

Description of feature

When creating a new Pipeline, it is possible to set a specific git branch name as default branch. Historically the default in git has been "master" - but the git community moves away fro that.

This PR introduces the capability of the backend to handle custom default branch names #1959

ToDo

• enable setting defaulBranch for pipeline repo creation #1959 (comment) for reference.