[RuntimeAsync] Move runtime async to the main repo

[VSadov]

Do not review just yet. May need some work.

A list of things to address. Some of these may be more relevant to address separately later.

Required before merge:

• Resolve build issues.
• Resolve test failures in existing tests
• Asses the performance impact from the presence of the feature.
• Make sure no new GC Stress failures.
• Restore CODEOWNERS
• Determine what to do about CompatibilitySuppressions
• Decide whether we want a feature ifdef and merge this disabled, or whether we can do the work upfront to merge it enabled
• Ensure new files have license headers
• (JIT) Add function headers on new functions
• Make sure crossgen2/ilc do not blow up on async2 methods
• Undo NativeCodeVersion/NativeCodeVersionNode/PrepareCodeConfig slot changes (not used in the end)
• Undo codeman.cpp white space change
• Figure out if eetwain.cpp change should be undone
• Figure out if gcinfodecoder.cpp change should be undone
• Remove auto in ClassifyAsyncMethodSignatureCore
• Add proper contracts and declarations for jitinterface.cpp methods for building resumption stubs
• Delete restoreregs_for_runtimesuspension.h
• Address stack underflow changes in stubgen.cpp
• Remove added ILCodeStream::EmitUNBOX if it is unused
• Make zapsig.cpp bool consistent with surrounding code
• Undo QCallHandles.cs change
• Decide whether meaningful names of getAsyncResumptionStub should be debug-only

Clean ups that are potentially not necessary for this PR (but some of these may be nice-to-have before review):

• (JIT) Remove support for capture-by-value for this in structs. Maybe ok to just have an assert/failfast instead, for now.
• (VM) Unify new OSR overload with existing overload
• (JIT/VM) Decide whether to remove new CORINFO_HELP_THROWEXACT in favor of using ExceptionDispatchInfo
• (JIT) Split up async2 transformation into smaller documented parts
• (JIT) Replace GenTreeCall::gtIsAsyncCall with a flag or calling convention change
• (JIT) Best-effort LA64/RISCV64 support
• (JIT/VM) Remove the "StructMethodThatOperatesOnCopy" mechanism (in favor of Roslyn-created copy)
• (JIT) Remove unnecessary intrinsic recognitions from special await helpers and get_RuntimeAsyncViaJitGeneratedStateMachines
• (VM) Rewrite MethodTableNative::GetLoaderAllocatorHandle in managed code
• (VM) Decide whether async thunks should be allowed to tier
• (VM) Optimize MethodTable::GetParallelMethodDesc