Backend: Add raw attribute to ci/cd settings variables & expose in endpoint

This is the backend issue to unblock Frontend: Make it possible to set variables as ... (#217309 - closed)

Problem to solve

Currently, the majority of variables sent by GitLab to Runner are subject to variable expansion and does not support special characters such as $. When users add a variable value with $ (which is very common for generated passwords), the value is treated as a variable reference, which isn't expected behavior.

Users find it surprising that the variable gets expanded by default when the value contains a $. Often times the value is machine generated and might contain a $ to make it more secure.

Proposal

Description	MR Link	Notes
Add raw to database	!88554 (merged)
Add VariableType to GraphQL
Add the variable field to Project, Group, Query