Introduce an R2R File Format change that can contain multiple assemblies

[mjsabby]

Today ReadyToRun PE files cannot have multiple assemblies in them. The feature request is that we build the capability to have multiple assemblies inside a single ReadyToRun PE file.

I'm opening the issue to track this, but @trylek is already working on a PR to document the changes in dotnet/coreclr#25344.

[MeiChin-Tsai]

@sergiy-k let's keep this for the .NET 5 prioritization. I would like see a justification.

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