Improve JSON validation in start-proxy action

[mbg]

This PR makes three changes related to the start-proxy action:

1. Additional validation is performed on the structure of the parsed JSON, to ensure that we have an array of configurations, configurations themselves are objects, and we don't accidentally treat null values as meaningful.
2. The backend may give us a configuration where the password is actually a token, which can lead to confusion when inspecting the logs and seeing that token is undefined even when it is expected to be defined. We believe that username is always undefined in that case, so we can use that as an indicator to treat the password as the token instead.
3. The test coverage is extended for these changes.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.