Add a logUpstreamErrors passthrough option

[theluckystrike]

Fixes #148

This adds the logUpstreamErrors option you suggested on #148, defaulting to true. Set to false on a passthrough rule, it stops the console output for upstream failures, which are often expected in test runs. Leaving it unset changes nothing.

server.forAnyWebSocket().thenPassThrough({
    logUpstreamErrors: false
});

The WebSocket passthrough is where the noise comes from, so that's where the gating is. In websocket-step-impls.ts it covers pipeWebSocket, mirrorRejection and the upstream request handlers in the step itself. The option lives on the shared passthrough connection options, so request passthrough rules accept and serialize it too, but that path doesn't print upstream connection errors today and its behaviour is unchanged. Both steps serialize the option and default it to true when deserializing, so remote rules work the same way.

Two new tests in test/integration/websockets.spec.ts trigger an upstream WebSocket failure through the proxy and capture the console. With the option off nothing is logged, and with the default the error still is. With the gating taken out of the build, the first test fails on the Error in proxied WebSocket line. The full websockets spec passes (34 tests).

Michael