Plugin template OpenAPI validation

# Summary

Currently, the structure of a plugin template is free-from, it can even be a string:

```yaml
plugin:
hello: "yes"
```

Thats fine. But we could allow plugin developers to define the OpenAPI schema and validate it when the template is created.

# Use Cases

When would you use this?

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**Message from the maintainers**:

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The issue names no files, tests, or entry points. Start by locating the plugin template creation path and any existing OpenAPI validation support, then clarify where plugin schemas should be defined and what validation errors and successful creation should look like.

Written by the indexing model from the issue text.