[primarily a placeholder for now for planning]

This adds invoke capability to function pointers, layering on See also #69273 and #75347.

Overloads will be added to ILGenerator.EmitCalli() to support new calling conventions that are modifier-based instead of enum-based. The enum-based calling conventions through System.Runtime.InteropServices.CallingConvention have not scaled well thus specifying "CallConv" types from the System.Runtime.InteropServices is assumed. Currently these include:

• CallConvCdecl
• CallConvStdCall
• CallConvThiscall
• CallConvFastcall
• CallConvSuppressGCTransition
• CallConvMemberFunction

The new APIs may likely take a proposed MethodSignature class from #75347.