Allow static Expression injection for instance methods

[akarboush]

Following up on #50 (Add custom InjectLambdaAttribute provider), I've just noticed that the current implementation doesn't allow mixing static and non static methods when injecting,

nein-linq/src/NeinLinq/InjectLambdaSignature.cs

Lines 49 to 51 in 0c5d7c1

	// mixed static and non-static methods?
	if (factory.IsStatic != isStatic)
	throw FailFactory(target, factory.Name, $"{(isStatic ? "static" : "non-static")} implementation expected");

which is often needed when working with external code. I think if we want to make this feature more useful, we should allow the expression to be static, since the method that needs to be translated will most likely be an instance method.