feat: support derived type mappings

[latonz]

Closes #330

• implements support to map derived types

[TimothyMakkison]

This is a really cool addition, I love how elegant the generated code is.

I wasn't sure where to put this but after reading the original issue I got thinking about adding generic attributes without breaking anything. Because generic attributes are a C#11 feature it mighr be possible to support them for compatible programs by doing as follows:

• Set the language version of Riok.Mapperly.Abstractions to 11
• Create a copy of MapDerivedTypeAttribute with 2 generic parameters and an additional empty constructor.
• Inside DerivedTypeMappingBuilder concat configs to a call to below.
• Programs using C# 10 or lower can use the typeof attribute constructor, getting an error if they try to use the generic attribute. C#11 programs can use either constructors.

// Check is MapDerivedTypeAttribute...

var attrDatas = symbol.GetAttributes().Where(x => x.AttributeClass.TypeArguments.Count() == 2);

foreach (var attrData in attrDatas)
{
    var namedSymbol = attrData.AttributeClass;
    yield return new MapDerivedType(namedSymbol.TypeArguments[0], namedSymbol.TypeArguments[1]);
}

I've hacked together a working version here, it appears to work with ,NET Framework and Core. I've probably overlooked something, perhaps it only works on modern .NET Sdk's or recent versions of Visual Studio. Hopefully this helps 🤞

[github-actions]

🎉 This PR is included in version 3.0.0 🎉

The release is available on:

• GitHub release
• v3.0.0

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