http-relay-server: Add a flag to set inactive request timeout

[drigz]

Please review the individual commits to separate the refactor from the new flag:

• refactor: use Config struct for server.go
• feat: add flag to change request timeout

We've seen outages where a critical request goes from taking slightly less than 60s to a bit more, and it's hard to response to these as the timeout is hard-coded. This should give us the flexibility to change it.

[ensonic]

Thanks. As a followup we can use the Option pattern. Then we have the setting directly in the server struct and avoid the long derefs at runtime (for both readability and performance)

[ensonic]

You'll need to also adjust the in_process_relay_test.go:

src/go/tests/relay/in_process_relay_test.go:51:19: not enough arguments in call to server.NewServer
	have ()
	want (server.Config)
src/go/tests/relay/in_process_relay_test.go:52:22: too many arguments in call to relayServer.Start
	have (int, int)
	want ()