schema: add "_vcollation" sysview

Add "_vcollation" sysview to read it from net.box. This
sysview is always readable, except when a user doesn't have
"public" role.

Needed for #3941

@TarantoolBot document
Title: box.space._vcollation

_vcollation is a system space that represents a virtual view.
The structure of its tuples is identical to that of _collation.
Tuples of this sysview is always readable, except when the user
doesn't have "public" role.