Enhance prep.sh for distro maintainers

[crummel]

In the past we've collected all of the RID-specific artifacts into one source-build-artifacts package for use when bootstrapping a new platform. prep.sh has largely replaced the need for this (#2758) but there are some scenarios where having all the available RID-specific bits in one archive is more convenient, for instance building on multiple platforms while sharing the same source code and source artifacts. Some enhancements suggested to support these scenarios in prep.sh:

• Improve the --bootstrap flag to allow downloading artifacts for specific RIDs.
• Allow users to supply their own set of packages for custom RIDs.
• Handle other RID-specific artifacts like the bootstrap SDK and coreclr-tools.

cc @dotnet/distro-maintainers

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