Better support for large solutions

[martinothamar]

Might be a poor title, just trying to aggregate some enhancement requests that revolve around large solutions where the current source generation approach seems insufficient. Things to consider for 3.0 relating to this

• Dynamically add handlers - i.e. there should be a way to be explicit about which messages and handlers are added
• Allow handlers to be internal - i.e. messages are public contracts, handlers are hidden

Related issues:
#94, #79, #62, #51

[martinothamar]

Implemented explicit assembly configuration in #202 (README is updated, will be available in 3.0.0-preview.61):

services.AddMediator((MediatorOptions options) =>
{
    options.Assemblies = [typeof(Ping)];
});

[martinothamar]

Related discussion thread: #231