add: secure cookie middleware

[thearchitector]

Summary

Add https://github.com/thearchitector/starlette-securecookies as another third party middleware.

Not sure what the exact process is for adding to / updating the docs, so let me know if something else is required!

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[adriangb]

Might want to rewrite it to be a pure ASGI middleware: https://www.starlette.io/middleware/#pure-asgi-middleware

[thearchitector]

It's already on the todo list, thanks!