Hi, Currently when using a user defined [custom_extensions](https://docs.rs/rcgen/latest/rcgen/struct.CertificateParams.html#structfield.custom_extensions), this causes the [CSR parsing](https://docs.rs/rcgen/latest/rcgen/struct.CertificateSigningRequest.html#method.from_der) to error out with [UnsupportedExtension](https://docs.rs/rcgen/latest/rcgen/enum.RcgenError.html#variant.UnsupportedExtension). Would it be possible to allow CSR parsing of custom extensions, such as by providing a closure to handle unknown extensions?