Raw data in x509.Certificate does not equal RawLogEntry.Cert.Data for precertificates

Hi there,

this is a follow-up to #947.

## Description
I want to generate hashes of certificates. To do that I use the raw data (bytes) stored in `x509.Certificate.Raw`. For [`x509.Certificate`](https://pkg.go.dev/github.com/google/certificate-transparency-go/x509#Certificate) generated from [`Precertificate`](https://pkg.go.dev/github.com/google/certificate-transparency-go#Precertificate) entries I am not getting the full (raw) certificate data in the `x509.Certificate.Raw` field.

Going one layer up, the `Precertificate` field `Submitted` contains the full submitted data which is also what I expected to be contained within the `x509.Certificate.Raw` field. Instead the `Raw` field seems to contain the unsigned certificate, which I don't fully understand. Eventually, for the TBS cert there is a whole other field (`RawTBSCertificate`). Also the Signature field is empty for Precertificates.

Looking at the Precertificate docs for the field `Submitted`:
>// DER-encoded pre-certificate as originally added, which includes [...] a signature generated over the pre-cert by the pre-cert issuer (which might differ from the issuer of the final cert, see RFC6962 s3.1).

#### Regular entry:
The ct.RawLogEntry.Cert.Data does equal the [`LogEntry`](https://pkg.go.dev/github.com/google/certificate-transparency-go#LogEntry).

#### Precertificate entry:

## Question
Is it intended that Precertificates do not contain the full DER-encoded data including the generated signature in the Raw field, but instead we must fall back to the `Precertificate.Submitted` field? I thought it would be more consistent and easier to have the raw certificate data in one place and not multiple ones.