Add brotli compression for windows

[kfreezen]

Doneness:

• Build is okay - I made sure that this change is building successfully.
• No Bugs - I made sure that this change is working properly as expected. It doesn't have any bugs that you are aware of. -- This supports only Windows. All other platforms currently throw a PlatformNotSupportedException for brotli.
• Branching - If this is not a hotfix, I am making this request against master branch

[AppVeyorBot]

✅ Build Titanium-Web-Proxy 3.0.693 completed (commit 3ba35157be by @kfreezen)

[gustyr]

Hi, do you know when a new nuget package will be available with this fix? :)

[AppVeyorBot]

✅ Build Titanium-Web-Proxy 3.0.694 completed (commit aea31c8aaa by @kfreezen)