[cdac] Make DAC load and use cDAC when available

[elinor-fung]

• If DOTNET_ENABLE_CDAC environment variable is 1, Look for cdacreader next to DAC and load it if found
• Implement ISOSDacInterface in cDAC - currently returns E_NOTIMPL for everything
  • This was kind of gross, but also the simplest way I could see of being able to slowly replace parts of the existing DAC while validating new functionality against existing.
• Make DAC delegate to cDAC (if available) for GetThreadStoreData and GetBreakingChangeVersion
• Initialize cDAC with function for reading from the target
  • Just tested reading a pointer manually by setting an address to read in the debugger

Contributes to #99298
Resolves #99301

[dotnet-policy-service]

Tagging subscribers to this area: @tommcdon
See info in area-owners.md if you want to be subscribed.

[jkoritzinsky]

This current model would require each implementation in the existing DAC of the ISOSDacInterfaceX interfaces to be edited each time a new method is implemented in the cDAC. Could we implement this using a COM Aggregation object that implements all the interface methods by calling the cDAC first and then calling the DAC implementation instead of updating each method implementation in the DAC?

[elinor-fung]

object that implements all the interface methods by calling the cDAC first and then calling the DAC implementation instead of updating each method implementation in the DAC?

That was actually my original plan. After getting in there and seeing just how many methods there were and how many of them don't seem relevant any more, I wasn't sure it was worth it. But maybe we start to implement more and decide it is. I could go either way.