feat: Add region variable to alias submodule for provider v6 resource-level region

[somaz94]

The modules/alias submodule had no region variable, so callers relying on the root module's provider-v6 resource-level region support could not guarantee the alias and its associated resources land in the same region as the function. They silently fell back to the provider's configured region, causing a ResourceNotFoundException at apply time when the function lived in a different region.

This adds an optional region variable (default null) to modules/alias and passes it to all seven regional resources, mirroring the root module's existing pattern:

• data.aws_lambda_alias.existing
• aws_lambda_alias.no_refresh / aws_lambda_alias.with_refresh
• aws_lambda_function_event_invoke_config.this
• aws_lambda_permission.version_triggers / aws_lambda_permission.qualified_alias_triggers
• aws_lambda_event_source_mapping.this

The wrappers/alias passthrough and the terraform-docs README row are included. Backward compatible: default = null is a no-op for existing users, and the provider requirement is already >= 6.28 so no version bump is needed.

Validation (local, all green):

• terraform fmt -check -recursive: clean
• terraform validate (init -backend=false): Success
• tflint: exit 0
• terraform-docs (v0.20.0): README Inputs table in sync, region row added

closes #760