Make Signal, Backend generic on read/set bounds

[DiamondJoseph]

Allows Signals to be put with one datatype and return another.
e.g. for Aravis cameras, where a non-exhaustive enum may live in the python runtime, with a subset of fields that are required, while the signal may extend with additional values that may vary on a hardware/reboot level and are unrealistic to track exhaustively.

Required for #190 #239 closes #238 may close #229

[callumforrester]

Technically I think this works, I had a play here: https://mypy-play.net/?mypy=latest&python=3.12&gist=64e5a5ed3d68622e0ed390e7cadefc39
Feel free to add more test cases if you think of any.

However, this is a somewhat significant design change so I think we should ensure that others are comfortable with it, especially @coretl and @abbiemery. This may even warrant an ADR.

[callumforrester]

It's a shame to have this for all signals when R and S will be the same >90% of the time. I wonder if it would make things simpler to introduce a new type instead of coopting SignalRW? @coretl may have thoughts.

[coretl]

Decision from the Ophyd meeting:

• Use an Enum when you require it to be exhaustive, make connect fail if the enum strings do not match exactly
• When you require a subset or otherwise, then use a str and return choices in the describe dictionary in case you need to check programmatically

I have one syntactic idea to ease the second point, instead of:

class PandaInput(Enum):
    ZERO = "ZERO"
    ONE = "ONE"
    # but there will be other entries that are only known at runtime

class SeqBlock(Device):
    enable: SignalRW[PandaInput]

def fly(self):
    await self.seq.enable.set(PandaInput.ONE)

then we drop to a str type but say we will restrict it for typing purposes:

PandaInput = Literal["ZERO", "ONE"]

class SeqBlock(Device):
    enable: SignalRW[PandaInput]

def fly(self):
    await self.seq.enable.set("ONE")

[callumforrester]

Can this PR be closed now?

[DiamondJoseph]

I suspect we're going to come across wanting to do this again later, but the branch can be restored when we need it.