Public API in the plugins

[304NotModified]

I realized that I made a breaking change in #479, as there is a lot of code that public while that's isn't necessary. It makes it very hard to fix bugs or make improvements without breaking code.

E.g. in ServiceCollectionFinder.cs has a public constructor (ctor), so you could never change the dependencies 🫨

Possible options:

1. Fix all breaking changes, and add [Obsolete] for unneeded code - semver is important of course - but I don't know how to fix [ScenarioDependencies] in a project without StepBindings gives 'No method marked with [ScenarioDependencies] attribute found' (Reqnroll.Microsoft.Extensions.DependencyInjection) #477 then (I need the new dependency) - maybe something with an optional ctor argument? This will get ugly 😅
2. Try to avoid breaking changes in not-documented code, but if it's hard, break stuff. (this means, revert the deletion of BindingRegistryExtensions.cs in Fix Reqnroll.Microsoft.Extensions.DependencyInjection is searching in the wrong assemblies #479 and drop Remove deadcode in AutoFac plugin #480)
3. Assume that the not-documented code and code assume that some code is never called by users, change them to internal / private. Maybe change the minor version number for this change (2.3.x -> 2.4.x). This means, also merge Remove deadcode in AutoFac plugin #480