Allow tags for each data_disk

[rswrz]

Summary

Add support for per–data disk tagging by extending the data_disks schema with an optional tags map and merging those tags with the module-level var.tags when creating azurerm_managed_disk resources.

What Changed

• Extended data_disks object type to include an optional tags map (default {}).
• Updated documentation in README.md and variables.tf.
• Updated disk resource tagging logic to merge global and per-disk tags.

Why

Consumers may require disk-specific metadata (e.g., cost center, classification, backup policy) while maintaining a consistent baseline tag set across the module. This change enables flexible tagging without duplicating resources or modifying module structure.

Impact

• Backwards compatible: existing configurations continue to work unchanged.
• New capability: per-disk tags via data_disks[*].tags.
• Precedence rule: per-disk tags override var.tags on key conflicts.
• Edge case: if var.tags is ever set to null, merge() would fail (assuming var.tags is expected to be a map).