Clarify required endpoints

Removed TODO (still in master branch).
Removed "The following list of API endpoints should be supported", because that seems wrong, or confusing at best.
Moved 'singular noun' sentence down because it seemed out of place, and added missing account.
Reworded the metadata endpoint sentence.
Added section on 'Required and Optional API'. There is some redundancy with the metadata paragraph, but I felt it was appropriate.

icpc / ccs-specs