Consider adding nested block support as alternative to nested attribute

### Use Cases or Problem Statement

Currently, the OpenAPI provider spec generator will always default to [nested attributes](https://developer.hashicorp.com/terraform/plugin/framework/handling-data/attributes#nested-attribute-types) when generating resource/data source/provider schemas with structural/collection types. This however requires that a provider be implemented with protocol v6 support and may not be desired for provider developers who want to use this tool for migrating from SDKv2 to Framework.

The [provider code spec](https://developer.hashicorp.com/terraform/plugin/code-generation/specification#blocks) and [`tfplugingen-framework`](https://developer.hashicorp.com/terraform/plugin/code-generation/framework-generator#nested-blocks) already support blocks.

### Proposal

Implement support for generating [nested blocks](https://developer.hashicorp.com/terraform/plugin/framework/handling-data/blocks) which can be supported on protocol v5. We want to encourage using nested attributes whenever possible, so we could introduce this as an opt-in configuration per-resource.

### Example:
```yml
provider:
name: petstore

data_sources:
pet:
read:
path: /pet/{petId}
method: GET
schema:
# Object = SingleNestedBlock, Collection = ListNestedBlock, Set= SetNestedBlock
v5: true
```

There are likely some edge-cases we might not be able to handle or express fully with the code generator (such as maps), so we'll need to determine how best to surface this information to users.

### Additional Information

_No response_

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