Pass http headers and user agent when downloading release notes

[zorgiepoo]

Pass http headers and user agent when downloading release notes. A developer requested me offline that they wanted this. I will try to make a back port to 1.x too.

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 in the debugger that a custom http headers and default user agent was passed to building the URL request for release notes.

macOS version tested: 11.4 (20F71)

[zorgiepoo]

Will fix #83

[zorgiepoo]

@kornelski Let me know if you're able to review this (and #1873) (when have the time)