Inconsistency between PersistentStorageDelegate::SyncGetKeyValue and KeyValueStoreManager

[tehampson]

Problem

• Some implementation of PersistentStorageDelegate use KeyValueStoreManager which is not capable of giving the size of for the time being, which makes this obligation of SyncGetKeyValue impossible.
  • After performing a code audit no one is currently using the functionality to get the size, so this change is safe, but we need platforms to adhear to the this updated description.
• PersistentStorageDelegate::SyncGetKeyValue should fill up the buffer up to size when CHIP_ERROR_BUFFER_TOO_SMALL is returned
  • This allows cases where fw downgrade happens (due to OTA revert, security reason, or any other reason) to be able to read initial part of potentially critical data (assuming that data is stored in TLV format where structure of TLV is only ever expanded)

Proposed Solution

• Change PersistentStorageDelegate::SyncGetKeyValue API documentation such that we fill up the buffer as much as we can and no longer provide size of the buffer needed when returning CHIP_ERROR_BUFFER_TOO_SMALL