Only support the formalized pvi structure from PandA

[coretl]

At the moment a pvi structure has a pvi field which contains the PVs for that Device. This is unlike any of the other records with have a value field instead.

PandABlocks/PandABlocks-ioc#120 will change all of this so that:

• typeid is epics:nt/NTPVI:1.0
• entries are in value not pvi

This means we can:

• Rename PvaDictConverter to PvaPviConverter, and remove all the methods apart from value
• Use PvaPviConverter where we see typeid epics:nt/NTPVI:1.0
• Remove

  ophyd-async/src/ophyd_async/epics/_backend/_p4p.py

  Lines 62 to 72 in cbbb295

  	def metadata_fields(self) -> List[str]:
  	"""
  	Fields to request from PVA for metadata.
  	"""
  	return ["alarm", "timeStamp"]
  	def value_fields(self) -> List[str]:
  	"""
  	Fields to request from PVA for the value.
  	"""
  	return ["value"]

  and hardcode them where they are used

This will require PandA IOC to be updated in sync with this change, but this is fine as we are pre-1.0

[coretl]

To be done after June experiment