pyoxidizer: support code signing Newer versions of PyOxidizer feature built-in support for code signing. You simply declare a code signer in the Starlark configuration file, activate it for automatic signing, and PyOxidizer will add code signatures to signable files as it encounters them. This commit teaches our Starlark configuration file to enable automatic code signing. But only on Windows for the moment, as our immediate goal is to overhaul the Windows packaging. The feature is opt-in: you must pass variables to PyOxidizer's build context via `pyoxidizer build --var` or `pyoxidizer build --var-env` to activate code signing. Differential Revision: https://phab.mercurial-scm.org/D10684