Clean up the Sigstore module and the require statements

[rochlefebvre]

TODO rebase after #27 merges

Follows #27

• Creating a Gem::Sigstore namespace via a rubygems/sigstore.rb file that requires all of the files directly under rubygems/sigstore.
• Moving sign_extend and verify_extend under rubygems/commands, alongside sign_command and verify_command.
• Removing the now-superfluous empty Gem::Sigstore declarations in various .rb files.
• requiring rubygems/sigstore in our helper.rb file.