Adds and implements conventions for model builder configuration

[Grinderofl]

Adds the ability to configure ModelBuilder from outside OnModelCreating method of DbContext.

Previous versions of EF lacked a proper way of doing it, this implementation gives the ability to add multiple IModelBuilderConvention types for configuration to individual/differing DbContext types. Dependency Injection is also supported.

Also sort-of solves #2805.

services.AddEntityFramework()
    .AddDbContext<Context>(builder => builder.UseConvention<MyBuilderConvention>());


public class MyBuilderConvention : IModelBuilderConvention
{
    public void Apply(ModelBuilder modelBuilder)
    {
        // (...);
    }
}

[dnfclas]

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[AndriySvyryd]

@Grinderofl
Thanks for sending this PR. We've discussed it and arrived at the conclusion that the API needs to be more flexible to be useful for more scenarios, see #2992
This is something that we plan to address before RTM, but you are free to send another PR