Consider exposing a Go module for consuming OpenAPI provider spec generation logic programmatically

### Use Cases or Problem Statement

There are use-cases where a developer may want more fine-grain control of the OpenAPI spec generation process, or may want to combine the generator with their own custom Go logic.

Currently, a developer would need to write a tool that interacts with the `tfplugingen-openapi` CLI directly which currently doesn't expose any information about the process, outside of the resulting provider code spec 😄. The CLI interface was also not designed with this in mind, so it can be cumbersome to implement.

### Proposal

Expose a Go module that can be consumed by other developer tools that contains the same logic as the CLI (generating provider code spec, configuring w/ generator config, etc.)

> _Proposed package design + API is TBD 😄_

### Additional Information

_No response_

### Code of Conduct

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