Enable signing of .tar.gz files using SignTool

[ellahathaway]

Edited: While we likely can't directly sign the tarball, we can probably create a detached signature (like we do for the source tarball).

[mmitche]

I don't think signing the tarball directly is supported. The most we can do is create a detached signature (like we do for the source tarball). Probably worth it at some point but I don't think this is required for UB

[ellahathaway]

Sounds good, I'll remove this from the epic and the UB backlog.

[missymessa]

If this isn't for UB, going to close.