Filter user workspaces by project id in Workspaces GraphQL API

Problem statement

In #395517 (closed), we are implementing a UI that allows users to view and manage workspaces for the project they are seeing in the Repository view. We need a way of only retrieving a user’s workspaces for the current project.

Solution proposal

Add a filter to the current user workspaces GraphQL API that allows retrieving workspaces for a given project id.