Add support to disable signing python files

In Aspire, we are shipping a template with some .py files in it. These .py files are getting a signing block appended to them.

We are working around this in https://github.com/dotnet/aspire/pull/13005, but would like an official way of excluding .py files from being signed. Similar to the work done in https://github.com/dotnet/arcade/pull/15760, where not signing JS files can be opted in/out with a flag.

cc @missymessa @ellahathaway @mmitche @joperezr