[Bug]: @typespec/http-server-csharp - add bin and obj to .gitignore

### Describe the bug

The generated .gitigore should ignore bin and obj for .NET:

Generated .gitignore

```
# MacOS
.DS_Store

# Default TypeSpec output
tsp-output/
dist/

# Dependency directories
node_modules/
```

### Reproduction

Install tsp then `tsp init` with cs server and openapi.
Even if I change the tsp config to scaffold the project files, it still doesn't change the .gitignore to ignore the bin and obj directories.


### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
- [x] Check that this is a concrete bug. For Q&A open a [GitHub Discussion](https://github.com/Microsoft/typespec/discussions).
- [x] The provided reproduction is a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example) of the bug.