v2: Make the split between Signal and SignalBackend support tables

[coretl]

At the moment, we have this interface for Signal.read and Signal.describe:

ophyd/ophyd/v2/core.py

Lines 661 to 669 in 7612b2c

	@_add_timeout
	async def read(self, cached: Optional[bool] = None) -> Dict[str, Reading]:
	"""Return a single item dict with the reading in it"""
	return {self.name: await self._backend_or_cache(cached).get_reading()}
	@_add_timeout
	async def describe(self) -> Dict[str, Descriptor]:
	"""Return a single item dict with the descriptor in it"""
	return {self.name: await self._backend.get_descriptor()}

This only allows a backend to produce a single data key in the Descriptor and Event. We decided (with @danielballan) to explode tables to be multiple fields, as this would make them look like arrays. This means that instead of a single panda-seq1-table entry in the descriptor with dtype=object, we would want an entry for every column (like panda-seq1-table-repeats, panda-seq1-table-time1, etc.) with dtype=array.

To do this, SignalBackend.get_descriptor and Signal.get_reading will need to change from:

ophyd/ophyd/v2/core.py

Lines 377 to 383 in 7612b2c

	@abstractmethod
	async def get_descriptor(self) -> Descriptor:
	"""Metadata like source, dtype, shape, precision, units"""
	@abstractmethod
	async def get_reading(self) -> Reading:
	"""The current value, timestamp and severity"""

To something like:

 @abstractmethod 
 async def get_descriptor(self, name: str) -> Dict[str, Descriptor]: 
     """Metadata like source, dtype, shape, precision, units""" 
  
 @abstractmethod 
 async def get_reading(self, name: str) -> Dict[str, Reading]: 
     """The current value, timestamp and severity""" 

And all the backends (and the SignalCache) change to produce these dictionaries instead of single values