Add string-based Property API that does not require a type

[ajcvickers]

When creating a shadow property it is important to know the type of that property:

builder.Entity<Foo>().Property<int>("Id");

But if string API is actually being used to reference a CLR property, then it would be nice to be able to do this:

builder.Entity<Foo>().Property("Id");

This API would throw if the "Id" property is not found, but if it is found, then its type will be used.
This becomes especially nice when mapping fields, since this requires that the string API be used:

builder.Entity<Foo>().Property("_id");