Reduce techports flapping as a result of Monorail 10G link watchdog

The Monorail task will periodically restart if the 10G link with Tofino is not up https://github.com/oxidecomputer/hubris/blob/b2b4861c32716d76bfa80243de87bbb4c5ce7f27/task/monorail-server/src/bsp/sidecar_bc.rs#L476-L502 in an attempt to work around the problem described here. But when the watchdog restarts the task it causes the front technician ports to flap, which is confusing during rack integration testing. This watchdog should really only be active if the PCIe link with Gimlet is attached and ideally would only kick in if Dendrite is running.

The sequencer can use the I2C link with Tofino to poll the state of its PCIe controller, which allows narrowing the window during which the watchdog is active. In addition, explore if monitoring some Tofino state registers can be used to determine if Dendrite is running.

Start in task/monorail-server/src/bsp/sidecar_bc.rs around lines 476-502 and read the linked document describing the 10G link problem. Trace how the sequencer can poll Tofino's PCIe controller over I2C and investigate whether Tofino state registers indicate that Dendrite is running. Done means the watchdog is narrowed to the attached PCIe link, ideally only while Dendrite runs, without causing front technician ports to flap.