Add RID-aware AssemblyLoadContext package

In order to make it easier for people to use LibGit2Sharp from something like an MSBuild task, we should consider adding a separate package that contains a custom AssemblyLoadContext with logic can properly load the right assembly based on the RID of the system. I'm thinking this could be named something like LibGit2Sharp.AssemblyLoadContext.

Doing something like this would be we could update the package as needed whenever we have a change in the set of native binaries.

@tmat Would it be possible to do something like move the code you've written for SourceLink into this proposed package instead? It would be nice to have that in a centralized place for everyone to use, and it would remove the burden on you to know when the RID mappings needed to be updated.

まず、提案されているカスタム AssemblyLoadContext、issue で言及されている SourceLink の読み込みコード、および現在のネイティブバイナリの RID マッピングを確認します。完全なパッケージに何を含めるかを定義する前に、パッケージの境界と RID の更新をどのように維持するかを明確にします。