In the proxy configuration form of a Product, the user should be able to add a new backend API config.

We need to consider the workflow carefully since the user may expect or not that proxy config for staging env to change once a new backend API config is added.

We should keep in mind that the Backend API may not yet exist. So either we will allow for it to created as part of the same process or demand that user first go through the "New Backend API" flow to only then be able to add a Backend API Config based on that.

Dev notes

We can use the simplest implementation
Workflow is:

1. Add New Backend API
2. In Product page, link to Backend API
3. Choose the Backend API and add the path

!!!Be aware that this should be part of the backends page: one of the things that will be extracted from apicast configuration edit page.