Add new variable use_for_each so we can use for_each to create multiple resource instances.

[lonegunmanb]

Describe your changes

This module was created before Terraform introduce for_each, and according to the document:

If your instances are almost identical, count is appropriate. If some of their arguments need distinct values that can't be directly derived from an integer, it's safer to use for_each.

Since the azurerm_subnet resources were created from var.subnet_names and these subnets are not identical, we should use for_each instead of count. But for the users that already maintained existing resources by this module, migrate from count to for_each requires heavy manually state move operations with extremely caution.

This pull request replicated a new azurerm_subnet which used for_each, and we provide a new toggle variable named use_for_each, this toggle is a switcher between count set and for_each set. Now user can set var.use_for_each to true to use for_each, and users who're maintaining existing resources could keep this toggle false to avoid potential breaking change.

Issue number

#000

Checklist before requesting a review

• The pr title can be used to describe what this pr did in CHANGELOG.md file
• I have executed pre-commit on my machine
• I have passed pr-check on my machine

Thanks for your cooperation!

[jiaweitao001]

LGTM. Thanks @lonegunmanb ！