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.

https://github.com/PandABlocks/PandABlocks-ioc/issues/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 https://github.com/bluesky/ophyd-async/blob/cbbb2950ed4c4b0bf4ca4c03729562b842b44b03/src/ophyd_async/epics/_backend/_p4p.py#L62-L72 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