[Projects] Add Get Project Donors Route (GET /projects/{id}/donors)

[nourshoreibah]

User Story

As a user I want to see all donors funding a project so that I can understand its financial sources.

Description

• Add GET route to projects Lambda.
  Command:
  node tools/lambda-cli.js add-route projects GET /projects/{id}/donors
• Join donor_proj with donors by projectID.
• Return each donor organization.

Acceptance Criteria

• Returns 200 with array of donors.
• Returns empty array if no donors.