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.

首先查看提议的自定义 AssemblyLoadContext、issue 中提到的 SourceLink 加载代码，以及当前的原生二进制 RID 映射。在确定完整 package 应包含哪些内容之前，先明确 package 的边界以及如何维护 RID 更新。