Have createNewBlock() return a BlockTemplate interface
Suggested in https://github.com/bitcoin/bitcoin/pull/29432#issuecomment-2225337100
An external program that uses the Mining interface may need quick access to some information in the block template, while it can wait a bit longer for the full raw transaction data.
This would be the case for a Stratum v2 Template Provider which needs to send a NewTemplate message message (which doesn't include transactions) as quickly as possible. It does not include the serialized block transactions.