Add a gas injection rig device

Based on the i18 spectroscopy work - it seems better to put this logic into a device

https://github.com/DiamondLightSource/spectroscopy-bluesky/pull/22

need to use the monitoring feature

```python
from ophyd_async.core import observe_value, observe_signals_value
line_pressure_iterator = observe_value(
gas_injector.pressure_controller_1.readout
)
value = await anext(line_pressure_iterator)

```

update: will also be used at b18 at some point https://github.com/DiamondLightSource/dodal/issues/1309