Add kms key creation for vault auto unseal

[itzkumaran]

Description

As of today, the KMS key creation process is manual outside the scope of the example - vault-auto-unseal and we are trying to bring this into Terraform. This adds Terraform source code for creating a KMS key for auto-unseal to work. This also provides a KMS key replication to another region.

Documentation

We have updated the documentation inside examples/vault-auto-unseal/README.md

TODOs

Please ensure all of these TODOs are completed before asking for a review.

• [x] Ensure the branch is named correctly with the issue number. e.g: feature/new-vpc-endpoints-955 or bug/missing-count-param-434.
• [x] Update the docs.
• [x] Keep the changes backward compatible where possible.
• [x] Run the pre-commit checks successfully.
• [ ] Run the relevant tests successfully.
• [x] Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.

Related Issues

Addresses #257

[hashicorp-cla]

All committers have signed the CLA.