[kchandan]

Part of #36 (step 1 of 4: the validator). Does not close it.

Adds the redis-migrate pack. launch reads a legacy Azure Cache for Redis, and the storage account if given, and reports blockers, warnings and info. It creates nothing. Rules are in labagent/packs/redis_migrate/rules.py as pure functions.

• Tested: 33 unit tests on hand-written cache documents (field names follow azure-mgmt-redis), and the pack with a fake az. The az commands were also run read-only against a real subscription (region list, storage list, redis list, unknown-cache error).
• Not tested: a real legacy cache. Microsoft blocks creating one for new customers, so the rules come from Microsoft's docs.
• Not covered yet: Enterprise sources, export, import, post-import checks (next PRs).
• No version bump in this PR.

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