Add support for host callback to get information for platform-native composite R2R images

[elinor-fung]

Depends on #120777. (First commit is a merge from that PR)

Add the basic runtime flow for calling into the host to get information about a platform-native R2R - see ReadyToRun Platform Native Envelope.

• Add get_native_code_data callback to host_runtime_contract
• If a component R2R assembly has READYTORUN_FLAG_PLATFORM_NATIVE_IMAGE, invoke the callback to get the R2R header, image size, and image base address
• Currently only implemented and tested on Windows (with local changes to include the flag)
• Mach-O support will need Add support for emitting Mach-O R2R images #121186

Contributes to #120065.

[jkoritzinsky]

cc: @adamperlin