Add a `api-version-parameter-strategy` option to openapi3 emitter

From design https://github.com/microsoft/typespec/issues/2819

```yaml
options:
@typespec/openapi3:
api-version-parameter: "flatten"
```

```tsp
@versioned(Versions)
@server("https://my-domain.com/api/{version}", "Main entry point", {version: Versions})
namespace MyNS;

enum Versions {v1, v2}
```

So it produce this
```
#--- v1.yaml
servers:
- url: https://my-domain.com/api/v1"
description: Main entry point
#--- v2.yaml
servers:
- url: https://my-domain.com/api/v2"
description: Main entry point
```