Allow configuring pinentry binary

[pks-t]

The pinentry binary can currently only be configured via the GnuPG configuration. On the one hand this is quite awkward on systems where GnuPG isn't used at all, as the user is now asked to create a file that shouldn't have anything to do with Goldwarden in order to configure it. And on the other hand this isn't really discoverable and doesn't exactly follow the principle of least surprise.

While it's nice that we try to honor a user's preexisting configuration, we should also provide a way to explicitly set the pinentry binary used by Goldwarden. This improves discoverability and also gives users more flexibility in case they want to use a different pinentry implementation for Goldwarden than for GnuPG, due to whatever reason.

Implement a new "PinentryBinary" configuration key. If set, it takes precedence over the value derived from the GnuPG configuration and over the platform-specific "pinentry-mac" default that is used on Darwin.