Fix project creation with templates using /projects/user/:id API

Merged Stan Hu requested to merge sh-fix-api-project-template-creation into master

Previously creation with an instance or group template with POST /api/v4/projects worked, but /api/v4/projects/user/:id did not because the latter did not accept the use_custom_templates parameter due to an oversight.

This commit fixes this by adding the optional_create_project_params Grape API and add tests for project creation via the user endpoint.

Closes #37004 (closed)

Edited by Mayra Cabrera