Initial Crossgen2 changes for composite R2R support (#31663)

Initial Crossgen2 changes for composite R2R support (#31663)

This change adds the new command-line option "--composite",
a new type of input files "--unrooted-input-file" for files that should
be made part of the composite image but only rooted as hit by the
dependency analysis, not automatically as the normal input files
(to reduce the size of framework built along with the app) and new
Crossgen2 logic for producing the additional R2R metadata needed
by composite images including rewriting of input MSIL by injecting
a "component R2R header" on them that forwards the native code
to the composite image. With a set of runtime changes out for
a separate PR I'm able to make first steps in running composite R2R
in the CoreCLR runtime.

Thanks

Tomas