Extension: library should provide a way to specify an external resource (Absolute URL)

[FredLL-Avaiga]

Description Say you want to use a js library available via a CDN (for example https://cdnjs.cloudflare.com/ajax/libs/frappe-gantt/0.6.1/frappe-gantt.min.js)

We should provide a way to set in the html: <script src="https://cdnjs.cloudflare.com/ajax/libs/frappe-gantt/0.6.1/frappe-gantt.min.js" integrity="sha512-HyGTvFEibBWxuZkDsE2wmy0VQ0JRirYgGieHp0pUmmwyrcFkAbn55kZrSXzCgKga04SIti5jZQVjbTSzFpzMlg==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>

Acceptance Criteria

• [ ] Ensure new code is unit tested, and check code coverage is at least 90%
• [ ] Propagate any change on the demos and run all of them to ensure there is no breaking change
• [ ] Ensure any change is well documented

[FredLL-Avaiga]

checking hostname via urlparse is good enough I guess