Display tags diff during diff and apply commands

[orien]

Context

Implements #302.

The stack_master diff and stack_master apply commands currently show "Stack diff" (template changes) and "Parameters diff", but do not display changes to stack tags. This makes it difficult to review tag modifications before applying changes to a stack.

Changes

• Extended StackDiffer to compute and display a "Tags diff" section alongside existing stack and parameter diffs
• Modified Stack.find to load current tags from CloudFormation stacks
• Rendered tags as one-line key: value pairs in YAML format for easy review
• Added feature test scenarios in diff.feature to verify tags are displayed with additions, removals, and modifications
• Added unit tests in stack_differ_spec.rb covering tag diff rendering
• Extended stack_spec.rb to verify tags are correctly loaded from CloudFormation
• Updated CHANGELOG to document the new feature

Both stack_master diff and stack_master apply now display tag changes in a consistent format, showing "Tags diff: No changes" when tags are unchanged or empty.