Use GenAPI to generate the targeting packs

Currently the targeting packs are based on IL and utilize IlAsm/IlDasm for building/generating the source. In theory we could utilize GenAPI to generate the targeting packs into c# code.

There are unknowns if GenAPI is mature enough to handle shared framework assemblies. A POC would be needed to prove this out.

APICompat should help validate this change.

The benefits of this would be:

1. Removing the IlAsm/IlDasm dependency. This adds additional bootstrap packages to the source build prep bootstrap process. e.g. this would help simplify the platform onboard process. This also unifies the build process between ref/targeting packs. Currently the targeting pack build infra is tied to Linux (see https://github.com/dotnet/source-build-reference-packages/issues/1322)
3. Consolidate the generate tooling to be based on one technology (e.g. GenAPI). Currently the targeting pack generator is not even in the public repo (see https://github.com/dotnet/source-build/issues/3091)