feat(trigger-http): add SPIN_HTTP_IDLE_INSTANCE_TIMEOUT_SECS env var

[TheRayquaza]

cc spinframework/containerd-shim-spin#426 for the intended usage

[itowlson]

@TheRayquaza @kate-goldenring Looking at the discussion on spinframework/containerd-shim-spin#426, I'm not clear if this is still needed, or if the shim will handle it by setting the CliArgs struct fields from code. (It feels a bit weird for the CLI to have to environment-variable-ify anything that SpinKube wants to set, but if that's where we are then so be it.)

If it is still needed, do we think there should be corresponding environment variables for other instance reuse parameters?

[TheRayquaza]

in fact not needed anymore 👌 closing