Enable trust serializer to unzip raw bytes from bytes.Reader

[bbengfort]

Right now the trust serializer expects an io.ReadCloser from zip.OpenReader when passed to serializer.Read() - this works when the zip file is being opened from a path on disk, but doesn't allow in-memory decompression of zip files, which may be required when using secrets storage such as Google Secret Manager or Vault. Although in most cases the secret will be mounted to a pod and read as though it were on disk, it would still be useful to have this utility.

• in serializer.Read in ZIP mode perform a type check to see if the reader is a zip.ReadCloser, a zip.Reader, or not.
• If zip.ReadCloser - continue as normal; if not a zip.Reader, create a zip.Reader from the reader.
• Process the zip.Reader independently of the zip.ReadCloser Using advice from this stackoverflow post

See also this small test issue in the directory repo.