FR: Support stdin as input file

[jbemmel]

For use in Unix pipelines:

<some command that generates YAML> | gnmic set --replace-path /configure/router[router-name=Base] --stdin yaml

We can discuss about the syntax; other tools use '-' as filename for stdin, the format could be auto-detected as well

[karimra]

The '-' syntax should be supported now. Did it fail for you?

[jbemmel]

I could not find it in the documentation, I did not try

[karimra]

We had this same conversation a couple of years ago: https://github.com/karimra/gnmic/issues/369