Adopt Swift Argument parser for Swift tools

[zorgiepoo]

Adopt Swift Argument parser for Swift tools: generate_keys, sign_update, generate_appcast
Before investigation of adding new options to generate_appcast, we should improve its argument parsing.

I referenced some bits of #1608

Fixes #1583

cc @Wevah

Checklist:

• My change is being tested and reviewed against the Sparkle 2.x branch. New changes must be developed on the 2.x development branch first.
• My change is being backported to master branch (Sparkle 1.x). Please create a separate pull request for 1.x, should it be backported. Note 1.x is feature frozen and is only taking bug fixes, localization updates, and critical OS adoption enhancements.
• I have reviewed and commented my code, particularly in hard-to-understand areas.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• My change is or requires a documentation or localization update

Testing

I tested and verified my change by using one or multiple of these methods:

• Sparkle Test App
• Unit Tests
• My own app
• Other (please specify)

I tested command line invocations (all potential options) for sign_update, generate_keys, and generate_appcast and tested invalid combinations of options to make sure they were rejected. For simplicity, I dropped the deprecated DSA argument support for generate_appcast in favor of using the DSA argument option (-f).

macOS version tested: 11.5.1 (20G80)