Fix issues with standard updater controller initialization

[zorgiepoo]

• Fix updater SPUStandardUpdaterController delegate / user driver delegate not being set when hooked up via IBOutlets in Interface Builder - we need to defer initialization of updater until awakeFromNib, after the outlets are set
• Disallow setting updater / user driver delegates programmatically after updater controller has been initialized by turning the outlets into instance variables

Related: #1957

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)

• Tested creating a SPUStandardUpdaterController programmatically (in Swift) and setting updater / user driver delegates in initializer - confirmed they fire.
• Tested creating a SPUStandardUpdaterController in a nib (in Swift project) and setting updater / user driver delegate outlets in IB - confirmed delegate methods fire.
• Tested when outlets are connected in a nib from an older version of Sparkle, they are still connected using this newer build of Sparkle.

macOS version tested: 11.5.2 (20G95)