HTTP/2 pings don't work if the connection lifetime is zero

[MihaZupan]

We have this optimization in the connection pool manager where we'll avoid storing connections that have the lifetime set to 0.

runtime/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnectionPoolManager.cs

Lines 65 to 75 in f0f1457

	// As an optimization, we can sometimes avoid the overheads associated with
	// storing connections. This is possible when we would immediately terminate
	// connections anyway due to either the idle timeout or the lifetime being
	// set to zero, as in that case the timeout effectively immediately expires.
	// However, we can only do such optimizations if we're not also tracking
	// connections per server, as we use data in the associated data structures
	// to do that tracking.
	bool avoidStoringConnections =
	settings._maxConnectionsPerServer == int.MaxValue &&
	(settings._pooledConnectionIdleTimeout == TimeSpan.Zero ||
	settings._pooledConnectionLifetime == TimeSpan.Zero);

This also controls whether we create the heartbeat timer, which does the HTTP/2 ping sending and timeout enforcement.
Since we don't account for KeepAlivePingDelay being set, a config that sets Lifetime/Timeout to 0 will also ignore the ping options.

Technically a bug, but the use cases where you'd care about this are likely very niche.

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