dotnet-getdocument support OpenAPI YAML format generation

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

[dotnet-getdocument](https://github.com/dotnet/aspnetcore/blob/main/src/Tools/dotnet-getdocument/README.md) is the tool that is used by [Microsoft.Extensions.ApiDescription.Server](https://learn.microsoft.com/en-us/aspnet/core/fundamentals/openapi/aspnetcore-openapi?view=aspnetcore-9.0&tabs=visual-studio%2Cvisual-studio-code#generate-openapi-documents-at-build-time) with Microsoft.AspNetCore.OpenApi to generate documentation at build time.

Currently, the tool can generate only JSON OpenAPI spec.
It would be nice to see `YAML` output as well, which has been commonly used with the `Swagger` generator.

### Describe the solution you'd like

Support a flag to specify output format, for example
```
--format yaml
```
or similar to `dotnet swagger`
```
--yaml
```

### Additional context

_No response_