fix: Remove slash from api_url

[YolanFery]

When pushing a pipeline using the SDK, I was getting http://localhost:8000//workspaces/test/pipelines/simple-etl which contains an extra slash. This PR fixes the issue, adds a unit test to cover the change and some parts of pipelines_push method.

Changes

• Strip the slash in the url method
• Reuse the api_url method to compute the public api url
• Add a unit test to cover the push method
• Refactor the unit tests to avoid duplication of code and constants
• Add the Pr template from other repos

How/what to test

• Pushing a pipeline works with and without slash at the end of the config url
• Unit test are still passing