[Rgen] Update the transformer code generator.

[mandel-macaque]

We are adding a few things:

1. Change the way we write the flag properies. Make them readonly and init them when the attr dict is added.
2. The attr dict property now sets all the flags so that we do not access the dict all the time.
3. Add a new extension method that will allow to retrieve attr data structs from the attr dict.
4. Add a new attr to mark those attribute names that have a attr data struct associated to them.
5. Add an AppendRaw implementation for targets older than dotnet9.

We are not yet generating the properties to retrieve the attr data to keep the PR small.

A sample of the generated code can be found here:
https://gist.github.com/mandel-macaque/b2eb57d6e0b9f90550cf5dcc8773fe18

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[vs-mobiletools-engineering-service2]

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[vs-mobiletools-engineering-service2]

✅ API diff for current PR / commit

.NET ( No breaking changes )

• iOS: vsdrops gist ( No breaking changes )
• tvOS: vsdrops gist ( No breaking changes )
• MacCatalyst: vsdrops gist ( No breaking changes )
• macOS: vsdrops gist ( No breaking changes )

❗ API diff vs stable (Breaking changes)

.NET ( ❗ Breaking changes ❗ )

• iOS: vsdrops gist ( ❗ Breaking changes ❗ )
• tvOS: vsdrops gist ( ❗ Breaking changes ❗ )
• MacCatalyst: vsdrops gist ( ❗ Breaking changes ❗ )
• macOS: vsdrops gist ( ❗ Breaking changes ❗ )

ℹ️ Generator diff

Generator Diff: vsdrops (html) vsdrops (raw diff) gist (raw diff) - Please review changes)

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🚀 [CI Build] Test results 🚀

Test results

✅ All tests passed on VSTS: test results.

🎉 All 121 tests passed 🎉

Tests counts

✅ cecil: All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (iOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (MacCatalyst): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (macOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (Multiple platforms): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (tvOS): All 1 tests passed. Html Report (VSDrops) Download
✅ framework: All 2 tests passed. Html Report (VSDrops) Download
✅ fsharp: All 4 tests passed. Html Report (VSDrops) Download
✅ generator: All 5 tests passed. Html Report (VSDrops) Download
✅ interdependent-binding-projects: All 4 tests passed. Html Report (VSDrops) Download
✅ introspection: All 4 tests passed. Html Report (VSDrops) Download
✅ linker: All 44 tests passed. Html Report (VSDrops) Download
✅ monotouch (iOS): All 10 tests passed. Html Report (VSDrops) Download
✅ monotouch (MacCatalyst): All 15 tests passed. Html Report (VSDrops) Download
✅ monotouch (macOS): All 12 tests passed. Html Report (VSDrops) Download
✅ monotouch (tvOS): All 8 tests passed. Html Report (VSDrops) Download
✅ msbuild: All 2 tests passed. Html Report (VSDrops) Download
✅ xcframework: All 4 tests passed. Html Report (VSDrops) Download
✅ xtro: All 1 tests passed. Html Report (VSDrops) Download

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