feature: introduce a more robust validation mode

[hbagdi]

Currently, decK provides a validate command which does JSON schema validation of the input and also creates an in-memory state. While this detects some errors with user-input, we can do more.
Kong ships with a validate endpoint for each entity:
https://docs.konghq.com/2.0.x/db-less-admin-api/#validate-a-configuration-against-a-schema

A new mode for validate command (gated using a boolean CLI flag) should run all the entities present in the state against the Admin API endpoint. This should catch almost all errors with the user input.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.