feature: opm alpha render-veneer support for pipe stdin

[grokspawn]

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

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