[Feature Request] Support for custom extensions for csr

I have a use case that need to generate a csr with customized extensions. While feeling painful to migrate away from cfssl, I would like to know whether it is possible to support this feature here.

```
INPUT: profile / .csr.json
OUTPUT: csr pem with custom extensions
```

Edit Note: Knowing that it could be unwise to support custom extensions for cert, would still want to submit pull request for codes in case there are people willing to work on such custom csr.

Start by tracing how the profile or .csr.json input is handled during CSR generation and where the CSR PEM output is assembled. Determine the supported representation and validation rules for custom extensions, then define completion as generating a CSR PEM that preserves the requested extensions and has coverage for the behavior.

Written by the indexing model from the issue text.