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.

先檢視提議的自訂 AssemblyLoadContext、issue 中提到的 SourceLink 載入程式碼，以及目前的原生二進位檔 RID 對應。在定義完整 package 應包含哪些內容之前，先釐清 package 的邊界，以及應如何維護 RID 更新。