feat: add exams link to header menu

[bra-i-am]

This pull request adds support for conditionally displaying an "Exams" dashboard link in the site header, based on the ENABLE_EXAM_DASHBOARD configuration flag.

Feature addition:

• Added logic in Header.jsx to display an "Exams" link in the navigation menu when config.ENABLE_EXAM_DASHBOARD is true. The link uses the URL from config.EXAM_DASHBOARD_MFE_BASE_URL.
• Added a new message definition for the "Exams" link in Header.messages.jsx for internationalization support.

How to test

1. Run a Pearson environment with Tutor or devstack
2. Ensure you have an MFE cloned to test this header component, and run it npm install to install the dependencies
3. Then run npm i @edx/frontend-component-header@github:Pearson-Advance/frontend-component-header#vue/PADV-1973 to install this branch in your MFE
4. Ensure your site configs look like this; in this case, I'm running everything over the default lms.main.localhost:18000 site:

{
    "course_org_filter": [
        "PX"
    ],
    "COURSE_CATALOG_API_URL": "http://discovery.main.localhost:18381/api/v1/",
    "ECOMMERCE_PUBLIC_URL_ROOT": "http://ecommerce.main.localhost:18130/",
    "ECOMMERCE_API_URL": "http://ecommerce.main.localhost:18130/",
    "SESSION_COOKIE_DOMAIN": ".main.localhost",
    "MFE_CONFIG": {
        "LMS_BASE_URL": "http://lms.main.localhost:18000",
        "LOGIN_URL": "http://lms.main.localhost:18000/login",
        "LOGOUT_URL": "http://lms.main.localhost:18000/logout",
        "REFRESH_ACCESS_TOKEN_ENDPOINT": "http://lms.main.localhost:18000/login_refresh",
        "ENABLE_EXAM_DASHBOARD": true,
        "EXAM_DASHBOARD_MFE_BASE_URL": "http://lms.main.localhost:2005",
        "WEBNG_PLUGIN_API_BASE_URL": "http://lms.main.localhost:18000/webng_plugin/api/v0"
    }
}

5. The ENABLE_EXAM_DASHBOARD variable will display the Exams link, and the EXAM_DASHBOARD_MFE_BASE_URL will set the redirect value when it is clicked
6. Restart the MFE and LMS so the settings and the new header component are integrated properly

Evidence

Exams MFE

Profile MFE