Allow app to control calls to `io_uring_submit()` in external execution mode with iouring

[anrossi]

Calls to io_uring_submit() with iouring do a user/kernel context switch, which is computationally expensive. When in external execution mode with iouring datapath, allow the app to do all calls to io_uring_submit, and have don't have MsQuic call io_uring_submit.

First steps would be seeing the current performance costs of having MsQuic call io_uring_submit and confirming that there's performance gains to be had.

[guhetier]

If we do something here, we need to either build an abstraction that makes sense for all queue types, or think hard about how we are going to build "datapath specific" / "queue type specific" API extensions.