v2.1 - Bugfixes, new abstractions for pre/post-processors and exception hadlers

Bugfixes:

• Fix thread safety issue in ISourceGenerator/IIncrementalGenerator implementations (#56) (also backported to 2.0.x)
• Make sure transient handlers are disposed (#73) (also backported to 2.0.x)
• Fix build performance regression after #77 (#84)
• Disable CS8321 warning for generated code, "unused local function" #85
• Harden AddMediator configuration - explicit type parameter (#82)

Feature:

• Pre, post processors and exception handlers (#67)
• Fix for the source generator to process projects that do not directly use any of the types from Mediator.Abstractions (#77) (thanks @anadale!)
  • Supports codegen for projects that transitively depend on Mediator.Abstractions

Other:

• Cleanup and modernization of ASP.NET Core clean architecture sample #12f2102