Add `ForConstraint` also to the `GivenSelector<T>`

[ITaluone]

Background and motivation

Now, it isn't possible to chain a .ForConstraint() after you have a .Given(() => xxx) in the chain.

This is somewhat related to #1918...

API Proposal

public class GivenSelector<T>
{
    public GivenSelector<T> ForConstraint(OccurrenceConstraint constraint, Func<T, int> func);
}

Additionally, to support inline string interpolation (especially, if the actual count should appear without quotes) I propose another .FailWith():

public class GivenSelector<T>
{
    public ContinuationOfGiven<T> FailWith(Func<T, string> message);
}

The reason is, that you then could write

.FailWith(" {expectedOccurrence}{reason}, but found it {0}.",
       actualCount  => actualCount.Times()));

like

.FailWith(actualCount  => "{expectedOccurrence}{reason}, but found it {actualCount.Times()}.");

This could also be internal, but in case I propose it as public.

assertionChain
      .ForCondition(Subject is not null)
      .BecauseOf(because, becauseArgs)
      .FailWith("...")
      .Then
      .WithExpectation("Expected something {0}", expected,
          chain => chain
              .BecauseOf(because, becauseArgs)
              .Given(() => Subject.Collection)
              .ForCondition(collection=> collection is not null)
              .FailWith("{reason}, but...")
              .Then
              .Given(collection=> collection.Count())
              .ForConstraint(occurrenceConstraint, actualCount => actualCount ) // <--- This one here
              .FailWith(" {expectedOccurrence}{reason}, but found it {0}.",
                  actualCount  => actualCount.Times()));

Alternative Designs

A quick but dirty alternative would be to call .ForCondition() like: .ForCondition(actual => occurrenceConstraint.Assert(actual)), but this seems hacky and you don't have the registered {expectedOccurrence} context data.

Risks

Non of I can think of...
The only drawback is, that in a case like above the passed in value func is formatted, and thus a string appears with qoutes in output.

Are you willing to help with a proof-of-concept (as PR in that or a separate repo) first and as pull-request later on?

Yes, please assign this issue to me.