Potential fix for code scanning alert no. 1: Server-side request forgery

[xDido]

Potential fix for https://github.com/advanced-computer-lab-2023/CodeMedics-Clinic/security/code-scanning/1

To fix the SSRF vulnerability, we need to validate and sanitize the user-controlled inputs (patientUsername, appointmentId, and packageName) before using them in the URL. Specifically:

1. Use an allow-list to restrict valid values for patientUsername, appointmentId, and packageName.
2. Ensure that the constructed route does not allow path traversal or other malicious manipulations.
3. Refactor the code to use server-controlled values instead of directly incorporating user input into the URL.

The changes will be made in Frontend/el7a2ny-frontend/src/components/CheckoutForm.js to validate the inputs before constructing the URL.

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