[yaml2obj, obj2yaml] - Implement section header table as a special Chunk.

This was discussed in D93678 thread.
Currently we have one special chunk - Fill.

This patch re implements the "SectionHeaderTable" key to become a special chunk too.
With that we are able to place the section header table at any location,
just like we place sections.

Differential revision: https://reviews.llvm.org/D95140