v2.0 - new configuration options, better error handling, bugfixes and perf improvements

Finally publishing the v2.0 release.

• Configure Mediator through AddMediator call (assembly attribute still works) (#21, #24)
• Throw ArgumentNullException for null messages (#22)
• Pass Publish<INotification> calls to Publish(object) (#22)
• Optimize notifications (52ns -> 11ns for the most common case) (#23)
• Cleanup and optimization in source generator (#25, thanks @Timmoth!)
• Modernize source generator to support both 3.8 and 4.0 Roslyn versions (also prepare for incremental codegen) (#26)
• Updated benchmarks to account for changes (#27)
• Smoketests, unittests, memory allocation tests (#28, #29)
• Cleanup analysis-part of source generator, use 6.0.0 of .NET references (#30)
• Use csharpier for consistent formatting (#31)
• Benchmark for source generation process (#32)
• Bugfix for using arrays as response types (#33, #34)
• Multiple bugfixes for stream requests and structs (see linked PRs #36, thanks @Tornhoof!)
• Improve error handling in source generation process (#33 (comment), #40)
• Support Notification Handler Identification via Reflection (#39, #42, thanks @Tornhoof!)
• Fix defensive copy for non-readonly structs (#43, #44, thanks @Tornhoof!)
• Add missing debugger attributes (#46)
• Bugfixes and benchmark for larger projects (#41)
• Fix thread safety issue in ISourceGenerator/IIncrementalGenerator implementations (#56, thanks for reporting @AlexDelepine!)
• Typo fix in README (#61, thanks @sjmdsky!)