API: Add endpoint to fetch all workspace invitations for a user

[rajdip-b]

Description

We would like to add an endpoint to fetch all the active workspace invitations that the user has.

Solution

• Add an endpoint /api/workspace/invitations under workspace.controller.ts
• Add a function named getAllWorkspaceInvitations in workspace.service.ts that implements the logic. The response should be paginated
• Add proper tests in workspace.e2e.spec.ts