Consider setting a finite ConnectTimeout by default

[MihaZupan]

Starting with 6.0, the consequences of a connection attempt that takes a long time to fail, or somehow becomes stuck are much higher than before (memory leaks, more failed requests ...).

In .NET 7, we made a change in the connection pool where we will now cancel a connection attempt 5 seconds after the request that initiated it completes (or is canceled).

The main mitigation for this in 6.0 is to set SocketsHttpHandler.ConnectTimeout to a finite value. We should consider setting such a default in YARP.
My preference would be that we make the change regardless of target framework (also change it for 7.0).

See #1833 (comment) for more context on this change.