gensupport/storage: allow Accept-Encoding header

**Is your feature request related to a problem? Please describe.**
In Storage, we use the `Accept-Encoding` header to allow users to download the zipped version of their data, see [`ReadCompressed`](https://pkg.go.dev/cloud.google.com/go/storage#ObjectHandle.ReadCompressed). We cannot support this in JSON because the header is disallowed.

**Describe the solution you'd like**
Allow the header, at least for storage media downloads. This requires changes to the generated code.

**Describe alternatives you've considered**
Allow the header in general. This can be done in gensupport. (edit: this actually would be my preferred solution, but it may impact other libraries - linked PR is for the first option)