feat: initial draft non blocking middleware provider

[mattsse]

Changes

• Adds a new SyncProvider type that works like a block working of a Middleware, it spawns the actual Middleware to a background thread and executes requests via channels

[mattsse]

Some initial testing indicates that this is indistinguishable from the BlockingProvider, but perhaps worth giving this another look once more parallel features are implemented

[gakonst]

Closing given unclear perf gains as discussed