Ad plugin stats

[DiamondJoseph]

Initial implementation of potential solution to #177

Adds a pair of example enums to the NDStatsPlugin, allows configuring appropriate fields for that plugin.
Allows configuring an ADBase with a dict of {suffix: PluginClass} that will then later enable configurable use of ScalarSigs in the HDFWriter

Submitting mostly as a RFC and first attempt: From the i22 pilatus and AdAravis cameras, the Stats plugin was hardcoded, without enabling the Statistics calculation, or allowing other fields. Ideally, this behaviour would be configurable from here?

[DiamondJoseph]

I don't know if this has already been discussed for ophyd-async, just something I stumbled across while porting the i22 cameras.

[DiamondJoseph]

I see that SingleTriggerDet is already taking a kwargs of Plugins

class SingleTriggerDet(StandardReadable, Triggerable):
    def __init__(
        self,
        drv: ADBase,
        read_uncached: Sequence[SignalR] = (),
        name="",
        **plugins: NDPluginBase,
    ) -> None:
        self.drv = drv
        self.__dict__.update(plugins)
    ....