[metadata] Assorted enhancements for some CIL image formats (#42398)

[metadata] Assorted enhancements for some CIL image formats (#42398)

Some  cherrypicked changes from a work in progress

* Try to set MonoImage guid from modules table for metadata_only images

   If the image has a modules table, and a guid heap, set MonoImage:guid when loading the image, even if it is metadata_only.  In minimal delta images this is true.

* Minimal delta files have 4 byte indices, always

* Don't populate MonoImage:module_name for minimal deltas

* Always store precise string heap size in MonoImage:heap_strings

   In minimal delta images, the String heap is appended to the heap of the base image.  But in the physical base image, the String heap is zero-padded to a size that's a multiple of 4 bytes.

   Compute the precise String heap size at base image load time.

Co-authored-by: lambdageek <lambdageek@users.noreply.github.com>