Use hashed rekord type when uploading to a Rekor instance

[LeSuisse]

This will avoid issues when signing large payloads and, in most cases, it is likely to be faster since the expensive checksum computation will be done locally and it will avoid having to upload the whole data to the Rekor instance

See sigstore/rekor#481 and sigstore/rekor#501 for more information.

[LeSuisse]

In fact this is not possible (yet?), the hashed rekord type can only validate x509 signature

https://github.com/sigstore/rekor/blob/40d9419f4b294277789480c89e407066f9b63734/pkg/types/hashedrekord/v0.0.1/entry.go#L147-L151