Add a test connection button when adding/editing a storage

[lorenzo-cavazzi]

We now support a broader range of external storage services. Configuring one isn't always straightforward, and we currently don't give feedback on wrong configurations until launching a new session. Even worse, some errors only appear when starting sessions, or in the running session when trying to access a mounted folder.

To improve the UX, we want to add a "Test connection" button on the modal to add/edit storage, so that users can immediately verify the configuration.

[lorenzo-cavazzi]

Closing this since there is a PR #2994 that will be included in the current ShapeUp project on saving the cloud storage credentials