Adds Test upstream button to Maven upstream form

Context

Once Virtual registries: add a test function to upst... (#535637 - closed) is closed we can add the Test upstream button to the Maven upstream form & integrate with the backend API.

Designs

• #493842[Create-upstream.png]
• #493842[Edit-upstream.png]

Proposal

Using patterns from integration settings,

• If the API is successful, show a toast message with the message Connection successful
• If the API fails, show a toast message with the message received from the server.