enhance the veneer rendering to be able to read the input veneer data from stdin, via a pipe, in a manner similar to https://dev.to/napicella/linux-pipes-in-golang-2e8j

then the command could be used in a manner similar to many k8s examples like

```shell
opm alpha render-veneer semver -o yaml < infile > outfile
```

Upstream issue link: https://github.com/operator-framework/operator-registry/issues/1011