[yaml2obj/obj2yaml] - Add support of 'Size' and 'Content' keys for all sections.

Many sections either do not have a support of `Size`/`Content` or support just a
one of them, e.g only `Content`.

`Section` is the base class for sections. This patch adds `Content` and `Size` members
to it and removes similar members from derived classes. This allows to cleanup and
generalize the code and adds a support of these keys for all sections (`SHT_MIPS_ABIFLAGS`
is a only exception, it requires unrelated specific changes to be done).

I had to update/add many tests to test the new functionality properly.

Differential revision: https://reviews.llvm.org/D89039