r/aws_billing_view: Fix data_filter_expression handling

[henkka]

This PR fixes a bug in the recently introduced aws_billing_view resource (initially added in #45097) where data_filter_expression configurations were being silently dropped during creation and update, leading to state inconsistencies.

Currently, if a user attempts to configure data_filter_expression (e.g., filtering by tags or dimensions), the generic flex reflection helpers fail to map the Terraform ListNestedBlock to the corresponding AWS SDK struct.

This results in two issues:

• Silent Data Loss: An empty DataFilterExpression struct is sent to the AWS API.
• State Inconsistency: Because the API returns no filter data (since none was sent), Terraform detects a diff between the plan (1 block) and the state (0 blocks) during the read phase.

resource "aws_billing_view" "this" {
  data_filter_expression {
    tags {
      key    = "tenant"
      values = ["example"]
    }
  }
}

produces

╷
│ Error: Provider produced inconsistent result after apply
│
│ When applying changes to aws_billing_view.this, provider "provider[\"registry.terraform.io/hashicorp/aws\"]" produced an unexpected new value:
│ .data_filter_expression: block count changed from 1 to 0.
│
│ This is a bug in the provider, which should be reported in the provider's own issue tracker.

To fix the issue, this PR introduces manual expansion and flattening logic for the data_filter_expression field.

• Expansion: Added expandDataFilterExpression helper to manually extract values from the Terraform ListNestedBlock (taking the 0th index) and construct the *awstypes.DataFilterExpression struct required by the AWS SDK.

• Flattening: Added flattenDataFilterExpression helper to correctly map the AWS SDK response back into the Terraform state structure.

[github-actions]

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[github-actions]

✅ Thank you for correcting the previously detected issues! The maintainers appreciate your efforts to make the review process as smooth as possible.