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 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.