Add policy check using Pulumi CrossGuard

[tachyonik]

Issue #5877

Scope

This pull request adds an option called --policy <path_to_policy_pack> to sst diff and sst deploy commands.

• Display policy violation errors
• Enforce deployment prevention due to breach of mandatory policies
• Complete example to demonstrate how the option works

The change allows users to use the new option in two different modes:

1. sst diff --policy /path/to/policy - view any policy violations
2. sst deploy --policy /path/to/policy - prevent deployment if there are policy violations

References:
Pulumi CrossGuard