Restored processors registration that was broken in MediatR 12.1.0+

[Anthony Simmon (asimmon)]

Description of changes

Context

We planed to leverage MediatR pre and post processors to execute arbitrary code before and after requests are executed.
These processors used to be automatically registered. Developers didn't have to manually register them in the DI services.

In MediatR 12.1.0, the automatic registration was removed, and devs were forced to manually register them in the DI services.
In MediatR 12.2.0, the automatic registration was supposed to be brought back under an option that must be set to true (opt-in), but it's bugged.

More details in this opened PR: LuckyPennySoftware/MediatR#989 (comment)

Changes

• Wrote a "hack" that calls the MediatR internal code to execute the pre/post registration as before. Basically has the same effect than the opened PR mentioned above.
• Added tests to ensure that we always automatically register pre/post processors and the corresponding behavior.

Breaking changes

Now that we upgraded to MediatR 2.2.0, there are a few side-effects:

• Microsoft.Bcl.AsyncInterfaces was upgraded from 6.0.0 to 8.0.0
• Same thing for was upgraded from 6.0.0 to 8.0.0

There's nothing we can do about that. There was no need for MediatR to do these upgrades. Usually third-parties dependencies like that are kept to a minimum version.

Additional checks

- [ ] Updated the documentation of the project to reflect the changes

• Added new tests that cover the code changes