Add separate framework for actix-web 4.0

[patrickfreed]

I'd like to submit a benchmark for the MongoDB Rust driver used along side actix-web, but the driver currently only works with actix-web 4.0, and the existing actix framework tests use an earlier version. I'd rather not go through all the effort of updating all the existing actix tests, so would it be okay if I introduced actix-4 as a new framework to test against for the MongoDB driver? Other contributors could then port the other tests to it too.

[robjtede]

the Actix Web tests in-tree now use v4 RC