Add Support for "Plan" Mode Before Applying Changes

[eye-dee]

Description

It would be highly beneficial if topicctl had the ability to perform a "plan" operation that displays the intended changes before actually applying them. This feature would align well with best practices seen in infrastructure-as-code tools like Terraform.

Motivation

1. Preview Changes: A "plan" mode would provide a detailed report of actions topicctl would take during an apply, enabling users to verify correctness before making irreversible modifications.
2. Safety: Reduces the risk of misconfigurations or accidental changes to critical topics.

Expected Behavior

• New Command or Flag: A --plan flag or topicctl plan command to preview changes.
• Output Format:
  • Show clear, human-readable descriptions of proposed changes (e.g., "Add partitions to topic X", "Update retention for topic Y").
  • Optionally support JSON or other machine-readable output formats for automation.

Example Usage

# Plan changes without applying them  
topicctl apply --plan --config-file topics.yaml  
  
# Separate command for planning  
topicctl plan --config-file topics.yaml